mark all SkImage methods const, so we can make it thread-safe

src/image/SkImage.cpp

Index: src/image/SkImage.cpp
diff --git a/src/image/SkImage.cpp b/src/image/SkImage.cpp
index c812f58eff6dc632f2df0ea4260ea2ea78390f48..795b8420e6e8cbdff7e3a8d635f79f1902ddc742 100644
--- a/src/image/SkImage.cpp
+++ b/src/image/SkImage.cpp
@@ -29,13 +29,12 @@ uint32_t SkImage::NextUniqueID() {
     return id;
 }
 
-void SkImage::draw(SkCanvas* canvas, SkScalar x, SkScalar y,
-                   const SkPaint* paint) {
+void SkImage::draw(SkCanvas* canvas, SkScalar x, SkScalar y, const SkPaint* paint) const {
     as_IB(this)->onDraw(canvas, x, y, paint);
 }
 
 void SkImage::draw(SkCanvas* canvas, const SkRect* src, const SkRect& dst,
-                   const SkPaint* paint) {
+                   const SkPaint* paint) const {
     as_IB(this)->onDrawRectToRect(canvas, src, dst, paint);
 }