Add private save-behind and draw-behind methods to canvas.HEADp9.0

The draw-behind  is a variant of drawPaint but is automatically clipped
to the bounds of the most recent saveBehind buffer (axis-aligned bounds).

No public exposure outside of the Android framework.

Impl is pretty simple (its a variant of drawPaint)
- find the most recent saveBehind device bounds
- if there is none, draw nothing, else
  - temporarily intersect the device's clip with that bounds
  - drawPaint
  - restore the clip

This patches did not apply cleanly and have been updated to compile
with a previous version of Skia. It was cherry-picked from the
following 3 Skia commits:
148b7fd3ad9c29dec0052de624c26ff291ef8f0a
d567408362bf7847d6000f6786f9a7b2c9d0b88b
9adc82c73df0ef25b708cae8aa48ef9c39ed4c67

Bug: 129117085
Test: None
Change-Id: I291f57885de6e95f749bf5cdb70ac16a5781ffb1

4 files changed, 28 insertions, 2 deletions

diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 2f83b66fdd..51c285fc08 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -739,7 +739,7 @@ public:
         Call restoreToCount() with returned value to restore this and subsequent saves.
 
         @param layerRec  layer state
-        @return          depth of save state stack
+        @return          depth of save state stack before this call was made.
     */
     int saveLayer(const SaveLayerRec& layerRec);
 
@@ -2484,6 +2484,8 @@ protected:
     virtual SaveLayerStrategy getSaveLayerStrategy(const SaveLayerRec& ) {
         return kFullLayer_SaveLayerStrategy;
     }
+    // returns true if we should actually perform the saveBehind, or false if we should just save.
+    virtual bool onDoSaveBehind(const SkRect*) { return true; }
     virtual void willRestore() {}
     virtual void didRestore() {}
     virtual void didConcat(const SkMatrix& ) {}
@@ -2520,6 +2522,7 @@ protected:
     virtual void onDrawDrawable(SkDrawable* drawable, const SkMatrix* matrix);
 
     virtual void onDrawPaint(const SkPaint& paint);
+    virtual void onDrawBehind(const SkPaint& paint);
     virtual void onDrawRect(const SkRect& rect, const SkPaint& paint);
     virtual void onDrawRegion(const SkRegion& region, const SkPaint& paint);
     virtual void onDrawOval(const SkRect& rect, const SkPaint& paint);
@@ -2674,6 +2677,7 @@ private:
     void internalSetMatrix(const SkMatrix&);
 
     friend class SkAndroidFrameworkUtils;
+    friend class SkCanvasPriv;
     friend class SkDrawIter;        // needs setupDrawForLayerDevice()
     friend class AutoDrawLooper;
     friend class SkDebugCanvas;     // needs experimental fAllowSimplifyClip
@@ -2694,6 +2698,22 @@ private:
     SkCanvas(const SkBitmap&, std::unique_ptr<SkRasterHandleAllocator>,
              SkRasterHandleAllocator::Handle);
 
+    /** Experimental
+     *  Saves the specified subset of the current pixels in the current layer,
+     *  and then clears those pixels to transparent black.
+     *  Restores the pixels on restore() by drawing them in SkBlendMode::kDstOver.
+     *
+     *  @param subset   conservative bounds of the area to be saved / restored.
+     *  @return depth of save state stack before this call was made.
+     */
+    int only_axis_aligned_saveBehind(const SkRect* subset);
+
+    /**
+     *  Like drawPaint, but magically clipped to the most recent saveBehind buffer rectangle.
+     *  If there is no active saveBehind, then this draws nothing.
+     */
+    void drawClippedToSaveBehind(const SkPaint&);
+
     void resetForNextPicture(const SkIRect& bounds);
 
     // needs gettotalclip()
@@ -2717,6 +2737,7 @@ private:
                                 SrcRectConstraint);
     void internalDrawPaint(const SkPaint& paint);
     void internalSaveLayer(const SaveLayerRec&, SaveLayerStrategy);
+    void internalSaveBehind(const SkRect*);
     void internalDrawDevice(SkBaseDevice*, int x, int y, const SkPaint*, SkImage* clipImage,
                             const SkMatrix& clipMatrix);
 
diff --git a/include/core/SkOverdrawCanvas.h b/include/core/SkOverdrawCanvas.h
index 284fe6cd97..eda33ddf41 100644
--- a/include/core/SkOverdrawCanvas.h
+++ b/include/core/SkOverdrawCanvas.h
@@ -31,6 +31,7 @@ public:
     void onDrawPatch(const SkPoint[12], const SkColor[4], const SkPoint[4], SkBlendMode,
                      const SkPaint&) override;
     void onDrawPaint(const SkPaint&) override;
+    void onDrawBehind(const SkPaint& paint) override;
     void onDrawRect(const SkRect&, const SkPaint&) override;
     void onDrawRegion(const SkRegion&, const SkPaint&) override;
     void onDrawOval(const SkRect&, const SkPaint&) override;
diff --git a/include/core/SkPicture.h b/include/core/SkPicture.h
index b2beb56b86..9452feeda9 100644
--- a/include/core/SkPicture.h
+++ b/include/core/SkPicture.h
@@ -168,10 +168,11 @@ private:
     // V59: No more LocalSpace option on PictureImageFilter
     // V60: Remove flags in picture header
     // V61: Change SkDrawPictureRec to take two colors rather than two alphas
+    // V62: Add saveBehind
 
     // Only SKPs within the min/current picture version range (inclusive) can be read.
     static const uint32_t     MIN_PICTURE_VERSION = 56;     // august 2017
-    static const uint32_t CURRENT_PICTURE_VERSION = 61;
+    static const uint32_t CURRENT_PICTURE_VERSION = 62;
 
     static bool IsValidPictInfo(const SkPictInfo& info);
     static sk_sp<SkPicture> Forwardport(const SkPictInfo&,
diff --git a/include/core/SkRect.h b/include/core/SkRect.h
index f9a5bff4ff..dd66d75076 100644
--- a/include/core/SkRect.h
+++ b/include/core/SkRect.h
@@ -154,6 +154,9 @@ struct SK_API SkIRect {
     */
     int32_t y() const { return fTop; }
 
+    // Experimental
+    SkIPoint topLeft() const { return {fLeft, fTop}; }
+
     /** Returns span on the x-axis. This does not check if SkIRect is sorted, or if
         result fits in 32-bit signed integer; result may be negative.