container: ignore borders in fullscreen windows

When setting the geometry from content for floating windows, the
coordinates for borders are normally taken into account. However in the
case of a floating fullscreen window, we should not be doing this. Since
the content of the container takes the space of the entire output, this
causes the calculated borders to neccesarily be outside of the output.
This later causes a problem when sending surface entrance events since
in a multi-monitor setup, the border coordinates will overlap with
another output despite the surface not actually being on that output at
all. The fix is to just ignore border coordinates for a floating
fullscreen container since fullscreen, of course, does not actually have
any borders. Fixes #6080.

sway/tree/container.c

@@ -894,7 +894,7 @@ void container_set_geometry_from_content(struct sway_container *con) {
 	size_t border_width = 0;
 	size_t top = 0;
 
-	if (con->pending.border != B_CSD) {
+	if (con->pending.border != B_CSD && !con->pending.fullscreen_mode) {
 		border_width = con->pending.border_thickness * (con->pending.border != B_NONE);
 		top = con->pending.border == B_NORMAL ?
 			container_titlebar_height() : border_width;