GetTexImageANGLE and GetRenderbufferImageANGLE use ReadPixels to
pull texture data. Luminance is not a renderable format, so it is
not supported by ReadPixels. To support this, override Luminance
formats to their underlying internal format.

Test: angle_end2end_test --gtest_filter="*GetTexImage*"
Bug: b/160014453
Bug: angleproject:4058
Change-Id: Id19344c2e2c06386a871338833e35b7747cb966b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2321740Reviewed-by: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>