Support multiple default versions, to enable ES vs. desktop contexts, for shaders missing a #version statement.

This also moved some parseContext code from the flex file to the proper C++ file.

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@20501 e7fa87d3-cd2b-0410-9028-fcbf551c1848