axis events (#62)

* input.h: add the wpe_input_axis_event_type_motion_smooth axis event type.

* input.h: add wpe_input_axis_2d_event that extnets the base wpe_input_axis_event struct with per-axis delta values.

* Rename wpe_input_axis_event_type_2d to wpe_input_axis_event_type_mask_2d.

Author: zdobersek

include/wpe/input.h

@@ -92,6 +92,9 @@ struct wpe_input_pointer_event {
 enum wpe_input_axis_event_type {
     wpe_input_axis_event_type_null,
     wpe_input_axis_event_type_motion,
+    wpe_input_axis_event_type_motion_smooth,
+
+    wpe_input_axis_event_type_mask_2d = 1 << 16,
 };
 
 struct wpe_input_axis_event {
@@ -104,6 +107,13 @@ struct wpe_input_axis_event {
     uint32_t modifiers;
 };
 
+struct wpe_input_axis_2d_event {
+    struct wpe_input_axis_event base;
+
+    double x_axis;
+    double y_axis;
+};
+
 
 enum wpe_input_touch_event_type {
     wpe_input_touch_event_type_null,