mesa: implement GL_ARB_map_buffer_range

mesa: implement GL_ARB_map_buffer_range

Only enabled for software drivers at this point.

Note that the gl_buffer_object::Access enum field has been replaced by
a gl_buffer_object::AccessFlags bitfield.  The new field is a mask of
the GL_MAP_x_BIT flags which is a superset of the old GL_READ_ONLY,
GL_WRITE_ONLY and GL_READ_WRITE modes.  When we query GL_BUFFER_ACCESS_ARB
we translate the bitfield into the conventional enum values.