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

src/image/SkImage.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkImagePriv.h"
@@ skipping 11 matching lines @@
     static int32_t gUniqueID;
 
     // never return 0;
     uint32_t id;
     do {
         id = sk_atomic_inc(&gUniqueID) + 1;
     } while (0 == id);
     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);
 }
 
 const void* SkImage::peekPixels(SkImageInfo* info, size_t* rowBytes) const {
     SkImageInfo infoStorage;
     size_t rowBytesStorage;
     if (NULL == info) {
         info = &infoStorage;
     }
     if (NULL == rowBytes) {
@@ skipping 84 matching lines @@
 
     SkCanvas canvas(*bitmap);
 
     SkPaint paint;
     paint.setXfermodeMode(SkXfermode::kClear_Mode);
     canvas.drawRect(dstR, paint);
 
     const_cast<SkImage_Base*>(this)->onDrawRectToRect(&canvas, &srcR, dstR, NULL);
     return true;
 }