SL-17551 Add "Select Reflection Probes" menu option and make invisible objects less annoying when alt-zooming in edit mode.

Add tinygltf import library, and partial gltf loader implementation.

Approved-by: Brad Kittenbrink

DRTVWR-559: Fix fullbrightShiny not compiling on AMD due to type mismatch

Approved-by: Dave Parks

DRTVWR-559 merge up to release v6.6.1

following promotion of DRTVWR-546

SL-17472 Add checkbox to preferences for enabling "PBR" that uses "class3" deferred shaders.  Move reflection probe shaders to class3 and restore class2 shaders to non-reflection probe versions.

SL-17287 Instrument and optimize cubemap render.  Fix for cubemap snapshots doing a full resolution render instead of a 512x512 render.

SL-17451 Fix for erroneous attempt to apply vertex color alpha to texture before alpha masking (we don't actually support this and the vertex color alpha is sometimes zero when you think it ought not be).