Make sure we don't calculate fractional pixel gaps

When gaps are resized for lack of space the calculation could result in
a gap size of non-integer pixels. This would result in containers
located at non-integer pixels which would be subtly broken.

sway/tree/arrange.c

@@ -71,7 +71,7 @@ static void apply_horiz_layout(list_t *children, struct wlr_box *parent) {
 	double total_gap = fmin(inner_gap * (children->length - 1),
 		fmax(0, parent->width - MIN_SANE_W * children->length));
 	double child_total_width = parent->width - total_gap;
-	inner_gap = total_gap / (children->length - 1);
+	inner_gap = floor(total_gap / (children->length - 1));
 
 	// Resize windows
 	sway_log(SWAY_DEBUG, "Arranging %p horizontally", parent);
@@ -150,7 +150,7 @@ static void apply_vert_layout(list_t *children, struct wlr_box *parent) {
 	double total_gap = fmin(inner_gap * (children->length - 1),
 		fmax(0, parent->height - MIN_SANE_H * children->length));
 	double child_total_height = parent->height - total_gap;
-	inner_gap = total_gap / (children->length - 1);
+	inner_gap = floor(total_gap / (children->length - 1));
 
 	// Resize windows
 	sway_log(SWAY_DEBUG, "Arranging %p vertically", parent);