In multi sample scenario, gl_SamplePosition is not handled with XY-flip and rotation, add codes for flip and rotation. Bug: angleproject:5637 Test: dEQP-GLES31.functional.shaders.multisample_interpolation. interpolate_at_offset.at_sample_position.default_framebuffer --deqp-gl-config-name=rgba8888d24s8ms4 Test: dEQP-GLES31.functional.shaders.sample_variables.sample_pos. correctness.default_framebuffer --deqp-gl-config-name=rgba8888d24s8ms4 Change-Id: I7ba2227e85c62d2ca7bf534a841a1c3a5c9beb5e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2781124Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Mohan Maiya <m.maiya@samsung.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
