Add tests for texelFetch with samplerExternalOES Add two tests to texelFetch in the vertex shader and fragment shader for YUV with samplerExternalOES. The fetch results are expected to be RGB value. Bug: angleproject:405149439 Change-Id: I0a5809b910ac61c678ab240aea8d303e0c9646e1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6382116 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Amirali Abdolrashidi <abdolrashidi@google.com> Auto-Submit: Aurora Zhang <Aurora.Zhang@arm.com> Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com>