use SkMakeBitmapShader to not force copies when possible

src/core/SkBitmapDevice.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmapDevice.h"
 #include "SkConfig8888.h"
 #include "SkDraw.h"
@@ skipping 304 matching lines @@
     } else {
         USE_DRAWBITMAP:
         // We can go faster by just calling drawBitmap, which will concat the
         // matrix with the CTM, and try to call drawSprite if it can. If not,
         // it will make a shader and call drawRect, as we do below.
         draw.drawBitmap(*bitmapPtr, matrix, dstPtr, paint);
         return;
     }
 
     USE_SHADER:
+
+    // Since the shader need only live for our stack-frame, pass in a custom allocator. This
+    // can save malloc calls, and signals to SkMakeBitmapShader to not try to copy the bitmap
+    // if its mutable, since that precaution is not needed (give the short lifetime of the shader).
+    SkTBlitterAllocator allocator;

[mtklein, 2016/08/05 15:46:43]

Perhaps as a follow up, I think this signal wants to become more explicit, e.g.
kWillNotOutliveStackframe.  It was already kind of cute to base this decision
off the presence of an allocator, but this allocator is the extreme, entirely
unused except as a bool.

[f(malita), 2016/08/05 15:52:50]

We already have ForceCopyMode for SkImageShader::Make - maybe plumb that on
SkMakeBitmapShader and use kNever_ForceCopyMode when we know the shader has
limited scope?

[reed1, 2016/08/05 16:45:12]

It is actually used to avoid mallocs when we alloc the actual shader.

     // construct a shader, so we can call drawRect with the dst
-    auto s = SkShader::MakeBitmapShader(*bitmapPtr, SkShader::kClamp_TileMode,
-                                        SkShader::kClamp_TileMode, &matrix);
+    auto s = SkMakeBitmapShader(*bitmapPtr, SkShader::kClamp_TileMode, SkShader::kClamp_TileMode,
+                                &matrix, &allocator);
     if (!s) {
         return;
     }
 
     SkPaint paintWithShader(paint);
     paintWithShader.setStyle(SkPaint::kFill_Style);
     paintWithShader.setShader(std::move(s));
 
     // Call ourself, in case the subclass wanted to share this setup code
     // but handle the drawRect code themselves.
@@ skipping 96 matching lines @@
         paint.getRasterizer() ||
         paint.getPathEffect() ||
         paint.isFakeBoldText() ||
         paint.getStyle() != SkPaint::kFill_Style ||
         !SkXfermode::IsMode(paint.getXfermode(), SkXfermode::kSrcOver_Mode))
     {
         return true;
     }
     return false;
 }