Apply the layer's image filter to the hoisted image

Exposing SkSurface_Gpu makes me sad and I would welcome alternatives.

This change is desireable since it greatly decreases the render target swaps.

Review URL: https://codereview.chromium.org/792923002
diff --git a/gyp/gpu.gyp b/gyp/gpu.gyp
index 6319456..e835e34 100644
--- a/gyp/gpu.gyp
+++ b/gyp/gpu.gyp
@@ -96,6 +96,7 @@
         '../include/gpu',
         '../src/core',
         '../src/gpu',
+        '../src/image/',
       ],
       'sources': [
         '<@(skgpu_sources)',
diff --git a/gyp/gpu.gypi b/gyp/gpu.gypi
index 32603fe..1570ab4 100644
--- a/gyp/gpu.gypi
+++ b/gyp/gpu.gypi
@@ -299,6 +299,7 @@
       '<(skia_src_path)/gpu/SkGrTexturePixelRef.cpp',
 
       '<(skia_src_path)/image/SkImage_Gpu.cpp',
+      '<(skia_src_path)/image/SkSurface_Gpu.h',
       '<(skia_src_path)/image/SkSurface_Gpu.cpp',
 
       '<(skia_src_path)/gpu/gl/SkGLContext.cpp'