Correct textureGather*() extension support:

Correct textureGather*() extension support:
 - add extension behavior and warning message for partial extension support
 - add partial support for gpu_shader5 for textureGather*
 - add interactions between texture rectangle and textureGather*
 - add checks to distinguish between gpu_shader5 and texture_gather extension feature differences

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@24183 e7fa87d3-cd2b-0410-9028-fcbf551c1848