swr/rast: clean up supported OGL4.0/4.1 extensions list
This commit adjusts the capabilities returned by the SWR driver and the documentation to correctly report the following extensions:
GL_ARB_texture_query_lod, GL_ARB_texture_cube_map_array, GL_ARB_gpu_shader_fp64, GL_ARB_texture_gather, GL_ARB_vertex_attrib_64bit.
No significant code changes are needed and all relevant piglit tests pass.