Fix the compressed texture row stride to a pixel stride in PxFmt

Also fix DXT1_RGBA block byte size in PxFmt.
Add SRGB DXT compressed texture support to Pxfmt library.
Verify support for various general state before restoring.

Further checks for both GL_ARB_imaging and GL_ARB_shader_image_load_store extension support prior to restoring related state.  These extensions not currently supported on Intel MESA.
Only query FBO params for "LAYERED" texture if GS extension supported

Currently not supported on Intel MESA.  Enum "GL_FRAMEBUFFER_ATTACHMENT_LAYERED" was introduced by GL_ARB_geometry_shader4 extension so dynamic check for this support prior to query seems reasonable.
Merge pull request #99 from jon-lunarg/github_07_15

Github 07 15