Before this commit, we always threw away the drawin's cairo surface whenever it
was made visible and thus forced a redraw.
This commit changes this so that we only force a redraw if the drawin was
resized why it wasn't visible. To remember when this happens, we free the
drawin's cairo surface without allocating a new one when the drawin is resized
while not being visible. Thus, we then only have to allocate a new surface if
the drawin doesn't have one when it is being made visible.
Signed-off-by: Uli Schlachter <psychon@znc.in>
346034c6b6