Increase GL2PS image error threshold to 15

This increases the GL2PS image error threshold from 10 to
15. This fixes the TestGL2PSExporterRaster tests on a few
dashboard machines.

Change-Id: Ic24c1f03d3e12aa69e64158e51bc9aae81fd7306
ENH: CXX tests should not fail if VTK_LARGE_DATA_ROOT is missing.

Change-Id: Ifef3f144080eaf887d135308ead0a57c7db86b2d
Merge topic 'increase-gl2ps-image-error-threshold' into master

c92103e0 Increase GL2PS image error threshold to 15
Merge topic 'TestWithoutLargeDara' into master

6b52b257 ENH: CXX tests should not fail if VTK_LARGE_DATA_ROOT is missing.