plugin: reset the filtered frame after sending it with buttons

Our filtered frame (frame without any button events) that gets passed
down to here may include relative motion or other events. Once we use
that frame to prepend the button event we need to reset it so we don't
cause duplicate motion and/or events with zero delta timestamps.

With the the previous code we got two identical frames:

  42: event25: plugin button-debounce        - 0.360 EV_MSC           MSC_SCAN               30
  ... event25: plugin button-debounce        - 0.360 EV_KEY           BTN_RIGHT               1
  ... event25: plugin button-debounce        - 0.360 EV_REL           REL_X                  -7
  ... event25: plugin button-debounce        - 0.360 EV_REL           REL_Y                  -7
  ... event25: plugin button-debounce        - 0.360 ----------------- EV_SYN ----------------- +8ms
  ... Plugin:button-debounce - debounce state: DEBOUNCE_STATE_IS_UP → DEBOUNCE_EVENT_OTHERBUTTON → DEBOUNCE_STATE_IS_UP
  ... Plugin:button-debounce - debounce state: DEBOUNCE_STATE_IS_UP → DEBOUNCE_EVENT_PRESS → DEBOUNCE_STATE_IS_DOWN_WAITING
  ... event25: plugin evdev                  - 0.360 EV_MSC           MSC_SCAN               30
  ... event25: plugin evdev                  - 0.360 EV_REL           REL_X                  -7
  ... event25: plugin evdev                  - 0.360 EV_REL           REL_Y                  -7
  ... event25: plugin evdev                  - 0.360 EV_KEY           BTN_RIGHT               1
  ... event25: plugin evdev                  - 0.360 ----------------- EV_SYN ----------------- +0ms
  ... Queuing  event25  POINTER_MOTION               +0.000s	-7.00/ -7.00 ( -7.00/ -7.00)
  ... event25 - middlebutton state: MIDDLEBUTTON_IDLE → MIDDLEBUTTON_EVENT_R_DOWN → MIDDLEBUTTON_RIGHT_DOWN, rc 1
  ... event25: plugin evdev                  - 0.360 EV_MSC           MSC_SCAN               30
  ... event25: plugin evdev                  - 0.360 EV_REL           REL_X                  -7
  ... event25: plugin evdev                  - 0.360 EV_REL           REL_Y                  -7
  ... event25: plugin evdev                  - 0.360 EV_KEY           BTN_RIGHT               1
  ... event25: plugin evdev                  - 0.360 ----------------- EV_SYN ----------------- +0ms
  ... Queuing  event25  POINTER_MOTION               +0.000s	 -nan/  -nan ( -7.00/ -7.00)
 event25  POINTER_MOTION               +0.360s	-7.00/ -7.00 ( -7.00/ -7.00)
 event25  POINTER_MOTION            2  +0.360s	 -nan/  -nan ( -7.00/ -7.00)

The 0ms delta caused a -nan in the custom pointer accel but even without
that bug the pointer would've jumped more than it should.

Closes #1172

(cherry picked from commit 4d317eae17)

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1326>

src/libinput-plugin-button-debounce.c

@@ -264,6 +264,11 @@ debounce_notify_button(struct plugin_device *device,
 	libinput_plugin_prepend_evdev_frame(device->parent->plugin,
 					    device->device,
 					    frame);
+
+	/* If we used the original frame, reset it to avoid re-sending any
+	 * non-button events that may be present in this frame */
+	if (button_frame == NULL)
+		evdev_frame_reset(frame);
 }
 
 static void