Fix various issues with framebuffer fetch

This CL does two things. First it fixes a bug of ours where when using
a custom fbFetch variable (es 3.0 and higher), we sometimes overwrite
the out color and then attempt to use it as the dst color later. This is
fixed here by using an intermediate variable.

Secondly I've added a workaround to an andreno fbFetch where reading from
the outColor always returns the original dstColor even if we've overwritten
the value.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2248403003

Review-Url: https://codereview.chromium.org/2248403003
diff --git a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
index 6845376..55ef9db 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
@@ -212,7 +212,7 @@
     void onFinalize() override;
     void defineSampleOffsetArray(const char* name, const SkMatrix&);
 
-    static const char* kDstTextureColorName;
+    static const char* kDstColorName;
 
     /*
      * State that tracks which child proc in the proc tree is currently emitting code.  This is