Allow glReadPixels() differences due precision errors

Allow glReadPixels() differences due precision errors

When comparing the results from glReadPixels() passing user-space memory
and using a PBO, there could be differences due drivers implementation
following different paths, and thus generating differences because of
precision errors.

Like in dEQP-GLES3.functional.read_pixels.* tests, add a threshold under
which the differences are acceptable.

Affects:
dEQP-GLES3.functional.pbo.*

Components: OpenGL

VK-GL-CTS issue: 1228

Change-Id: I7c9ac5de359cff8deba039b010357eae0da3eaaf