COMP: Increase error tolerance for TestGL2PSExporterVolumeRaster

After changing from Debug to Release, the Continuous had a small
variation in the produced image.

Change-Id: I382882429008597d96ab8c5e82b65a2863c09671
Merge topic 'Tolerance_PNGCompare' into master

d5524e91 COMP: Increase error tolerance for TestGL2PSExporterVolumeRaster