gpu, cmaa: blend color like bilinear filter

This CL fixes two points
* blend color like bilinear filter. Current implementation is strange hackish
  blend.
* Revert blurCoeff to the original value as the comments explain. The value
  might be changed to make hackish blend look good. After revert, CMAA looks
  much more similar to MSAA

BUG=535198
TEST=Run a WebGL app with Chrome started with "--enable-cmaa-shaders"
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2104003002
Cr-Commit-Position: refs/heads/master@{#402788}
1 file changed