glsl: Parse the allowed image format qualifiers in GLSL ES 3.1.

glsl: Parse the allowed image format qualifiers in GLSL ES 3.1.

This includes the minimum required desktop/ES GLSL version in the
format qualifier table in anticipation of new GLSL versions extending
the set of supported image formats.  According to section 4.4.7 of the
GLSL ES 3.1 spec:

"The format layout qualifier identifiers for image variable
 declarations are:
 [...]
 rgba32f
 rgba16f
 r32f
 rgba8
 rgba8_snorm
 [...]
 rgba32i
 rgba16i
 rgba8i
 r32i
 [...]
 rgba32ui
 rgba16ui
 rgba8ui
 r32ui"

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