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>