Mesa Software Renderer add-on don't support direct mode

[phoudoin]

Now that BGLView::DirectConnected() pass correctly clipped screen region to the delegated renderer, we should make some use of it in our defacto default/fallback renderer, the fully software one.

In single buffer opengl mode, we're supposed to blit/span/clear this screen region directly, while in double buffer we could spawn/awake at SwapBuffers() time a thread drawing (eventually doing dithering at fly) the bitmap buffer into the screen region.