Implement GL_ARB_shader_texture_image_samples.  Also add in gl_MaxSamples and the float imageAtomicExchange.

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@27721 e7fa87d3-cd2b-0410-9028-fcbf551c1848