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

1 files changed, 22 insertions, 1 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);