TODO: Why did this work for the equivalent non-rigged prim?

SL-18869 Touch up -- Prune "post_pos" from shadow shaders that don't need it and fix non-standard gl_Position z values

Fix alpha mask shadows to mult vert alpha prior to discard tests and obey min alpha settings.

Make shadow/shadowAlphaMask use consistent varying and output.

Also fixes bug with 0% and 1% transparency providing varying different visual results.

Fix OSX shader compile bug from use of #if FOO instead of #ifdef FOO or #if defined(FOO)
preventing atmospherics (and thus ALM) from working on OSX.

a bunch of trivial clean-ups and commentary.

Coalesced patchset.

SH-2181 Fix for alpha flickering when basic shaders enabled (don't use glAlphaFunc when shaders are available).

Reviewed by Leslie

SH-1682 Dynamically adjust the number of texture channels to use for indexed texture rendering based on available hardware.

See https://wiki.lindenlab.com/wiki/License_Substitution for details.

Self reviewed.

Merge of QAR-1267 to trunk.  This was a combo merge of QAR-1175 (maint-render-9) and QAR-1236 (dll-msvcrt-2)

svn merge -r 109838:112264 svn+ssh://svn.lindenlab.com/svn/linden/branches/maint-render/maint-render-9-merge-r109833