mesa: Add support for GL_NV_fill_rectangle

Since we don't have the bits required to support this in OpenGLES yet,
this only enables support for Desktop OpenGL

Signed-off-by: Lyude <lyude@redhat.com>

Changes since v1:
- Simply _mesa_PolygonMode() a little bit
- Fix formatting in OpenGL spec excerpts
- Move polygon mode checking into _mesa_valid_to_render()
Changes since v3:
- Improve error message for invalid drawings with GL_FILL_RECTANGLE_NV

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
4 files changed