Extend the image test to verify RGBX AHB and GL_RGB upload This changes updates the RGBXAHBImportThenUpload test to do texture upload to the right side and adds a new helper to validate the render result on both sides to check the upload respects the GL_RGB channels. Bug: b/238460927 Test: ImageTestES3.RGBXAHBImportThenUpload Change-Id: I61723e7db99dab78b7828b783bb7cdf062f39ad9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3849462 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Auto-Submit: Yiwei Zhang <zzyiwei@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>