Fix read size validation for RGBX formats. GL_RGBX8_ANGLE is the only format where the upload format is 3-channel RGB, whilethe download format is 4-channel RGBX. As such, the internal format corresponding to format+type expects 3-byte input/output. The format is fixed here for readPixels to output 4 bytes per pixel. Bug: chromium:1458046 Change-Id: Iec737ed64bade003cfab50dc5f595eb4875e81e4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4706957 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>