Add a new offset mode to the GL API XML. This mode, called "assign,"

Add a new offset mode to the GL API XML.  This mode, called "assign,"
tells the scripts to assign an available offset to the function.  The
important changes are in src/mesa/glapi/gl_XML.py and
src/mesa/glapi/*.xml.

Since the DRI drivers only depend on functions required by the ABI
(e.g., GL 1.2 + ARB_multitexture) having fixed offsets, all functions
not in the ABI use "assign" mode.  This has caused the offset of
basically every function outside the ABI to change.  I have verified
that a libGL with this patch works with a DRI driver without the patch.

Futher, several function were removed from the dispatch tables
altogether.  These are the functions for the following extensions:

	GL_SGIS_texture_filter4
	GL_SGIS_texture4D
	GL_SGIS_detail_texture
	GL_SGIS_sharpen_texture
	GL_SGIX_sprite
	GL_SGIX_instruments
	GL_SGIX_framezoom
	GL_SGIX_tag_sample_buffer
	GL_SGIX_reference_plane
	GL_SGIX_flush_raster
	GL_SGIX_list_priority
	GL_SGIX_fragment_lighting
	GL_PGI_misc_hints
	GL_EXT_index_material
	GL_EXT_index_func
	GL_3DFX_tbuffer

This removes 50 functions from the dispatch table.