core: opacity_old should be double

Using opacity_t for opacity_old resulting in opacity being rounded to 0.
This is fine until the opacity reaches 0, where the opacity != opacity_old
check fails and the damage is not added.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/compton.c

@@ -440,7 +440,7 @@ static win *paint_preprocess(session_t *ps, bool *fade_running) {
 		next = w->next;
 		const winmode_t mode_old = w->mode;
 		const bool was_painted = w->to_paint;
-		const opacity_t opacity_old = w->opacity;
+		const double opacity_old = w->opacity;
 
 		if (win_should_dim(ps, w) != w->dim) {
 			w->dim = win_should_dim(ps, w);