The shader directly used v_position.xy as output color, but for half of
the image x and y are negative and the respective channel became zero.

This change makes the shader use v_position.xy*0.5+0.5.  Additionally,
the tests that use this shader are changed to mathematically derive the
pixel colors instead of using magic numbers.

Bug: angleproject:5395
Change-Id: Ic11c362c22ca725aa173faa5fd6033a02a4303fd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2559265Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>