winex11.drv: Spring cleanup in X11DRV_setup_opengl_visual, and added one new visual config (with aux buffers).

The hwstretch blit code creates a new texture each time it is called
to back up the back buffer and releases it afterwards. It is more
efficient to keep the texture and release it with the surface.

OpenGL AUX buffers provide a way for offscreen rendering which is very
similar to our back buffer "offscreen" rendering emulation. Not all
card support aux buffers, but if they are available they are a nice
present which is easy to use.

Only return valid values for the location of the parameter if it is
the one requested. Currently returns address when ANY quoted or
bracketed parameter found.