add a field to struct sbus_device which records the framebuffer size reported
by ioctl(FBIOGATTR) so drivers can use it
remove some unnecessary #includes
support hardware acceleration via SX
This requires a recent cgfourteen kernel driver which allows userland to map
SX register space.
Basic acceleration is enabled by default ( use Option "Accel" "false" to turn
it off ), XRender support is incomplete but good enough for anti-aliased text
rendering ( enable with Option "xrender" "true" ).