use more xcb-util functions

[absolutelynothelix]

there are possibly other useful functions but these are the obvious ones.

x: remove x_screen_of_display and use xcb_aux_get_screen instead

x: remove x_sync and use xcb_aux_sync instead

x: remove x_get_visual_depth and use xcb_aux_get_depth_of_visual instead

x: don't require an entire struct x_connection in x_get_visual_for_depth

inspired by the xcb-util's xcb_aux_get_depth_of_visual function implementation

[yshui]

LGTM