glsl: implement layout qualifiers

glsl: implement layout qualifiers

For GL_ARB_fragment_coord_conventions.

This only applies to gl_FragCoord and controls pixel center origin and
pixel center integer.  For example:

layout (origin_upper_left, pixel_center_integer) varying vec4 gl_FragCoord;

This features introduces the idea of re-declaring variables with a changed
type.  This may also apply to arrays in some cases but that's not
implemented at this time.