Instead of creating a graphics context for every surface_t, this commit
adds a cache that allows to "remember" up to two GCs. Thus, the code
uses less GCs. When a GC from the cache can be used, this also gets rid
of a round-trip to the X11 server. Both of these are tiny, insignificant
savings, but so what?
Since GCs are per-depth, this code needs access to get_visual_depth().
To avoid a code duplication, this function is moved to libi3.
Fixes: https://github.com/i3/i3/issues/3478
Signed-off-by: Uli Schlachter <psychon@znc.in>
d6e2a38b5c