In order to fully suppot OES_shader_multisample_interpolation
rewrite interpolateAtOffset's offset parameter to account for
Y-coordinate flip.

Bug: angleproject:3589
Tests: dEQP-GLES31.functional.shaders.multisample_interpolation.
        interpolate_at_offset.*.default_framebuffer
Change-Id: I6bf72ceb1c0466ff5d4a92b72d1ec9e2552d6f2a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2524711
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>