SH-1521 FIX -- Lighting and Shadows grayed out on mac

* Modified mac feature table to bring it up to date with latest renderer features including deferred rendering
* Changed mac AGL pixel format to no longer allow it to revert to software rendering when shader compilation errors occur.
* Fixed up various GLSL shader compilation warnings and errors that came up as a result of the above changes.
* Changed initial conditions for the "hardware skinning" checkbox on the preferences floater so it can be modified before login on machines that support avatar vertex programs.
* Removed unused avatarAlphaF.glsl files

Reviewed by davep