Remove _mesa_ResizeBuffersMESA() call from _mesa_set_viewport().

Remove _mesa_ResizeBuffersMESA() call from _mesa_set_viewport().
Now, the driver's Viewport routine should call _mesa_ResizeBuffersMESA()
if necessary.
Cleaned up code related to GLframebuffer width/height initialization.
Set initial viewport/scissor params in _mesa_make_current2(), instead of
in the drivers' MakeCurrent functions.