If an app never called glViewport, the viewport size was always 0 by 0 pixels.
Now pass initial size to st_create_framebuffer() and initialize the viewport
and scissor bounds in st_make_current().
This could also be fixed by ensuring the gl_framebuffers passed to
_mesa_make_current() were initialized to the right size.  But that involves
allocating the renderbuffers/pipe_surfaces earlier and that runs into some
other issues ATM.
Also remove obsolete createRenderbuffers param to st_create_framebuffer().