make set3DMask virtual, so we can safely notify the shadercontext

also boost preallocated storage to account for this combo of bitmapshader + emboss + colorfilter

BUG=skia:
R=djsollen@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/563563002
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 8530023..6effe79 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -185,6 +185,9 @@
             return SkShader::CanCallShadeSpan16(this->getFlags());
         }
 
+        // Notification from blitter::blitMask in case we need to see the non-alpha channels
+        virtual void set3DMask(const SkMask*) {}
+
     protected:
         // Reference to shader, so we don't have to dupe information.
         const SkShader& fShader;