Add test cases of image built-in functions Add test cases that image accesses a mipmap level of texture, meanwhile exposes two driver bugs. Bug: angleproject:1987 TEST=angle_end2end_tests.ComputeShaderTest.* Change-Id: I4c43cdb97de8223d5e9af3757f41eb01eac81d0a Reviewed-on: https://chromium-review.googlesource.com/c/1415727 Commit-Queue: Xinghua Cao <xinghua.cao@intel.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>