enable fallback path
Add readback path for CopyTextureCHROMIUM
There are some liminations or driver bugs which cause GPU-GPU copy
unavailable in CopyTextureCHROMIUM. For such cases, we do a readback
fallback, i.e. draw the source texture to an intermediate texture in
RGBA format, read back pixels of the intermediate texture from GPU to
CPU, then upload pixels to destination texture with TexImage api. With
this approach, the caller of glCopyTextureCHROMIUM doesn't need to care
about the liminations and bugs mentioned above.
BUG=
612542
fix Y16 video texture
fix comments#4
Enable CopyTextureCHROMIUM in Blink for Tex{Sub}Image2D with more cases
BUG=
612542
TEST=webgl and webgl2 conformance tests
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel