fips-dispatch: Completely separate fips-dispatch GL prototypes from GL/gl.h

Move the OpenGL prototypes previously in fips-dispatch.h to a new
fips-dispatch-gl.h. The idea here is that any given file should
include only one of GL/gl.h or fips-dispatch-gl.h.

Files that implement libfips wrappers for OpenGL functions include
GL/gl.h to ensure that they implement functions with the correct
prototypes.

Meanwhile, files that call into OpenGL functions at run-time, (such as
metrics.c which calls the various OpenGL query-related functions),
instead include fips-dispatch-gl.h and do not include GL/gl.h. With
this approach, any new calls to OpenGL functions will cause
compilation warnings if the stubs are not also added to
fips-dispatch-gl.h.