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

src/image/SkImage_Gpu.cpp

Index: src/image/SkImage_Gpu.cpp
diff --git a/src/image/SkImage_Gpu.cpp b/src/image/SkImage_Gpu.cpp
index 81114b0fe8d1dfa59a17a9f1c148b4ed7f93f35d..0918412e83c1decbb2bad6e9d26dc8501d73ce55 100644
--- a/src/image/SkImage_Gpu.cpp
+++ b/src/image/SkImage_Gpu.cpp
@@ -20,12 +20,13 @@ public:
     explicit SkImage_Gpu(const SkBitmap&);
     virtual ~SkImage_Gpu();
 
-    virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) SK_OVERRIDE;
-    virtual void onDrawRectToRect(SkCanvas*, const SkRect* src, const SkRect& dst, const SkPaint*) SK_OVERRIDE;
-    virtual GrTexture* onGetTexture() SK_OVERRIDE;
+    virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y, const SkPaint*) const SK_OVERRIDE;
+    virtual void onDrawRectToRect(SkCanvas*, const SkRect* src, const SkRect& dst,
+                                  const SkPaint*) const SK_OVERRIDE;
+    virtual GrTexture* onGetTexture() const SK_OVERRIDE;
     virtual bool getROPixels(SkBitmap*) const SK_OVERRIDE;
 
-    GrTexture* getTexture() { return fBitmap.getTexture(); }
+    GrTexture* getTexture() const { return fBitmap.getTexture(); }
 
     virtual SkShader* onNewShader(SkShader::TileMode,
                                   SkShader::TileMode,
@@ -54,17 +55,16 @@ SkShader* SkImage_Gpu::onNewShader(SkShader::TileMode tileX,
     return SkShader::CreateBitmapShader(fBitmap, tileX, tileY, localMatrix);
 }
 
-void SkImage_Gpu::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y,
-                         const SkPaint* paint) {
+void SkImage_Gpu::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y, const SkPaint* paint) const {
     canvas->drawBitmap(fBitmap, x, y, paint);
 }
 
 void SkImage_Gpu::onDrawRectToRect(SkCanvas* canvas, const SkRect* src, const SkRect& dst,
-                         const SkPaint* paint) {
+                                   const SkPaint* paint) const {
     canvas->drawBitmapRectToRect(fBitmap, src, dst, paint);
 }
 
-GrTexture* SkImage_Gpu::onGetTexture() {
+GrTexture* SkImage_Gpu::onGetTexture() const {
     return fBitmap.getTexture();
 }