mesa: GL_ARB_map_buffer_alignment is not optional
Every driver supports it. All current and future Gallium drivers always
support it, and all existing classic drivers support it.
v2: Making GL_ARB_map_buffer_alignment a desktop OpenGL extension only.
v3: Squash two commits together.
v4 (idr): MIN_MAP_BUFFER_ALIGNMENT queries don't have any dependencies.
In previous versions of the patch it depended on EXTRA_API_GL which
would prevent the query from working in core profile contexts.
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>