Add test to verify the direct YUV sampling case This is a follow-up test to the commit below. https://chromium-review.googlesource.com/c/angle/angle/+/6797965 The test verifies that the yuv image and image view are properly initialized in a program using __samplerExternal2DY2YEXT sampler. Bug: angleproject:386749841 Test: angle_end2end_tests --gtest_filter=ImageTestES3.* Change-Id: Ia0142dfb9135df6684526bf88f8c8b257416be10 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/7001563 Commit-Queue: mohan maiya <m.maiya@samsung.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com>