glsl: Add default precision qualifiers to the symbol table

glsl: Add default precision qualifiers to the symbol table

The GLSL ES spec specifies default precision qualifiers for certain types,
so populate the symbol table with these.

Notice that the desktop GLSL spec also indicates defaults for some types
but this is not really useful since precision qualifiers are completely
ignored in desktop GLSL.

v2: simplify and add samplerExternalOES, specified by
    OES_EGL_image_external (Tapani)

v3: add atomic_uint (reported missing by Marta)

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>