ui/vnc: introduce VNC_DIRTY_PIXELS_PER_BIT macro

Signed-off-by: Peter Lieven <pl@kamp.de>
ui/vnc: optimize dirty bitmap tracking

vnc_update_client currently scans the dirty bitmap of each client
bitwise which is a very costly operation if only few bits are dirty.
vnc_refresh_server_surface does almost the same.
this patch optimizes both by utilizing the heavily optimized
function find_next_bit to find the offset of the next dirty
bit in the dirty bitmaps.

Signed-off-by: Peter Lieven <pl@kamp.de>
ui/vnc: disable adaptive update calculations if not needed

Signed-off-by: Peter Lieven <pl@kamp.de>