Relax mediump interpolation accuracy requirements and clarify thresholds

Relax mediump interpolation accuracy requirements and clarify thresholds

This change relaxes mediump interpolation accuracy in
dEQP-GLES3.functional.fragment_out.* to 4 ULP from 1 ULP.

Threshold computation is also fixed/clarified by moving interpolation
threshold to be dependent on precision qualifier, and allowing only
rounding difference in format-dependent threshold. The original error
was likely due to incorrectly accomodating for interpolation in format
threshold.

Bug: 19380820
Change-Id: I457a9b7e07bad94399d07f37caa29d65ad95a5c8