Change signatures of filter bounds methods to return a rect.

Change filterBounds(), onFilterBounds() and onFilterNodeBounds() and computeFastBounds() to
return the destination rectangle. There was no code path that could
return false, and returning rects by value is ok now.

BUG=skia:5094
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1823573003

Review URL: https://codereview.chromium.org/1823573003
diff --git a/src/effects/SkImageSource.cpp b/src/effects/SkImageSource.cpp
index c0a8dee..d63eda9 100644
--- a/src/effects/SkImageSource.cpp
+++ b/src/effects/SkImageSource.cpp
@@ -113,8 +113,8 @@
     return surf->makeImageSnapshot().release();
 }
 
-void SkImageSource::computeFastBounds(const SkRect& src, SkRect* dst) const {
-    *dst = fDstRect;
+SkRect SkImageSource::computeFastBounds(const SkRect& src) const {
+    return fDstRect;
 }
 
 #ifndef SK_IGNORE_TO_STRING