Use correct, stricter coverage variation thresholds in texture tests

Use correct, stricter coverage variation thresholds in texture tests

Cube map and 3D texture lookup validation utilities accommodate for
potential coverage computation differences by allowing pixels around
triangle edges to match either of the two triangles forming the test
quad.

The thresholds used eariler applied slightly too small bias, but in
wrong coordinate space (clip-space instead of window coords). This
change fixes both issues, making tests a bit more strict, and correct.

Bug: 19099914
Change-Id: Ife90c9a6a5261954e9f98b68e199248cfc589920