This test reproduces the cause of the failures with the
following dEQP tests:

dEQP.GLES31/functional_texture_multisample_samples_*_sample_position

This CL also fixes a problem with the previous test, and puts
the common rendering/ReadPixels code in a common method.

Bug: b/156395519
Change-Id: Id38476b6caf6f25c52f7504187fb500a39109438
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2207550Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>