seatop_down: Change type of sx, sy to double in begin_seatop_down

Losing the precision resulted in wlr_cursor and wlr_seat::pointer_state
getting out of sync during pointer motion in seatop_down.
Since the difference was always under 1 px, it was practically
impossible to notice in normal use.

But because of being out of sync, cursor_rebase would always end up
incorrectly calling wlr_seat_pointer_notify_motion from
seatop_default_begin (on releasing mouse button) which broke cursor
locking.

See #5405
Closes #4632

include/sway/input/seat.h

@@ -239,7 +239,7 @@ enum wlr_edges find_resize_edge(struct sway_container *cont,
 void seatop_begin_default(struct sway_seat *seat);
 
 void seatop_begin_down(struct sway_seat *seat, struct sway_container *con,
-		uint32_t time_msec, int sx, int sy);
+		uint32_t time_msec, double sx, double sy);
 
 void seatop_begin_move_floating(struct sway_seat *seat,
 		struct sway_container *con);

sway/input/seatop_down.c

@@ -90,7 +90,7 @@ static const struct sway_seatop_impl seatop_impl = {
 };
 
 void seatop_begin_down(struct sway_seat *seat, struct sway_container *con,
-		uint32_t time_msec, int sx, int sy) {
+		uint32_t time_msec, double sx, double sy) {
 	seatop_end(seat);
 
 	struct seatop_down_event *e =