GL: Enable OES_shader_multisample_interpolation Drive-by: * Fixed validity of the related state queries * Added SampleMultisampleInterpolationTest * Removed unused OES_sample_variables code Bug: angleproject:8097 Bug: angleproject:8131 Change-Id: I816d2096759a96d8691acd0a8c5ecdedf880201b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4440826 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>