mesa: Implement GL_ARB_polygon_offset_clamp

mesa: Implement GL_ARB_polygon_offset_clamp

Semantically identical to the EXT version (whose string is still valid
for GLES), so rename the bit but expose both extension strings.
(Suggested by Ilia Mirkin and Ian Romanick.)

v3: Fix the entrypoint alias in GL4x.xml (Ilia)

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>