The X11 server tells us about things that need to be redrawn via expose events.
When we get such an expose event before lua drew the drawable, we just fill the
exposed area with old data (which is black for newly-created drawables).
Fix this by tracking if we have any usable data in a drawable's double buffering
pixmap. This flag is unset whenever we throw away the old content (e.g. due to a
resize) and is set when lua gave us some new content to display.
Signed-off-by: Uli Schlachter <psychon@znc.in>
86f3b7f11f