Add double buffering with X DBE extension in hope to get rid of the
tearing issue. Thanks to cairo-compmgr for providing hints. Could be
enabled with --dbe. Only very limited tests have been done, I don't know
if it actually solves the tearing issue. My estimation is it is harmful
for performance, but I found no clear evidence. Experimental, so no
configuration file option is available for it.
MONITOR_REPAINT is broken if --dbe is turned on, this is intended for
testing whether DBE is actually working.
Richard Grenville
71613f20c2