Reintroduced manual framebuffer config selection.

The default behavior of WGL, EGL and GLX is to choose a config that has
/at least/ the specified number of bits, whereas the GLFW 2 behavior was
to choose the closest match with very few hard constraints.  Moving the
responsibility of finding the supported minimum values to the client was
problematic, as there's no way to enumerate supported configurations,
forcing the client to perform multiple (and slow) window/context
creation attempts.  Not even the currently set defaults (24-bit color
and depth, 8-bit stencil) is universally supported, as bug reports show.