* Add a binary cache for compiled shaders using glProgramBinary

* Add additional sanity checking to shader binary save and load, hook up cache clear and menu option

* Fix default init of shader cache data struct and clear gl errors before glGetError calls

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Co-authored-by: RunitaiLinden <davep@lindenlab.com>

SL-19655 Remove concept of "low priority" updates (RenderDebugGLSession assertion stomping pass).  Remove a couple unused debug settings and related code.

following promotion of DRTVWR-577

SL-19674 Disable "Blinn-Phong" when a PBR material is applied to make it clear that those buttons are inoperable until you remove the PBR material.

SL-19709 Fix for fullbright shiny not factoring out exposure and flickering fullbright alpha.  Incidental decruft.

This reverts commit d875f809.