Support OES_shader_multisample_interpolation extension if
maxInterpolationOffset >= 0.5

Bug: angleproject:3589
Tests: dEQP-GLES31.functional.shaders.multisample_interpolation.*
       dEQP-GLES31.functional.state_query.multisample_interpolation.*
Change-Id: I42997f10be82e3be8b63c56833cbbf791bf4be9b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2477905
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>