glsl: add a mechanism to allow #extension directives in the middle of shaders

glsl: add a mechanism to allow #extension directives in the middle of shaders

This is needed to make Unigine Heaven 4.0 and Unigine Valley 1.0 work
with sample shading.

Also, if this is disabled, the error message at least makes sense now.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>