input/seatop_down: add axis handler

This adds an axis handler to seatop_down so that it is possible to
manually scroll while having a mouse button down. This is mainly useful
for selecting text. Some applications may not automatically scroll when
the cursor is near the edge of the application or the user may just
prefer manually scrolling for more control over the scrolling speed.

sway/input/seatop_down.c

@@ -1,4 +1,5 @@
 #define _POSIX_C_SOURCE 200809L
+#include <float.h>
 #include <wlr/types/wlr_cursor.h>
 #include "sway/input/cursor.h"
 #include "sway/input/seat.h"
@@ -11,6 +12,20 @@ struct seatop_down_event {
 	double ref_con_lx, ref_con_ly; // container's x/y at start of op
 };
 
+static void handle_axis(struct sway_seat *seat,
+		struct wlr_event_pointer_axis *event) {
+	struct sway_input_device *input_device =
+		event->device ? event->device->data : NULL;
+	struct input_config *ic =
+		input_device ? input_device_get_config(input_device) : NULL;
+	float scroll_factor =
+		(ic == NULL || ic->scroll_factor == FLT_MIN) ? 1.0f : ic->scroll_factor;
+
+	wlr_seat_pointer_notify_axis(seat->wlr_seat, event->time_msec,
+		event->orientation, scroll_factor * event->delta,
+		round(scroll_factor * event->delta_discrete), event->source);
+}
+
 static void handle_button(struct sway_seat *seat, uint32_t time_msec,
 		struct wlr_input_device *device, uint32_t button,
 		enum wlr_button_state state) {
@@ -42,6 +57,7 @@ static void handle_unref(struct sway_seat *seat, struct sway_container *con) {
 }
 
 static const struct sway_seatop_impl seatop_impl = {
+	.axis = handle_axis,
 	.button = handle_button,
 	.motion = handle_motion,
 	.unref = handle_unref,