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>

docs/relnotes/17.3.0.html

@@ -44,6 +44,7 @@ Note: some of the new features are only available with certain drivers.
 </p>
 
 <ul>
+<li>GL_ARB_polygon_offset_clamp on i965, nv50, nvc0, r600, radeonsi, llvmpipe, swr</li>
 <li>GL_ARB_transform_feedback_overflow_query on radeonsi</li>
 <li>GL_ARB_texture_filter_anisotropic on i965, nv50, nvc0, r600, radeonsi</li>
 <li>GL_EXT_memory_object on radeonsi</li>