use SkMakeBitmapShader to not force copies when possible

src/core/SkBitmapDevice.cpp

Index: src/core/SkBitmapDevice.cpp
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index 21601f40372b004dea942545abc812789de67fa9..86bb49e69bef3175e17484b4f6094af0cef14c22 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -322,9 +322,14 @@ void SkBitmapDevice::drawBitmapRect(const SkDraw& draw, const SkBitmap& bitmap,
     }
 
     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;
     }