wayland: Update for relative transformation function support

src/video/wayland/SDL_waylandevents.c

@@ -3042,17 +3042,25 @@ static void relative_pointer_handle_relative_motion(void *data,
     struct SDL_WaylandInput *input = data;
     SDL_VideoData *d = input->display;
     SDL_WindowData *window = input->pointer_focus;
-    double dx_unaccel;
-    double dy_unaccel;
+    double dx;
+    double dy;
 
     // Relative pointer event times are in microsecond granularity.
     const Uint64 timestamp = SDL_US_TO_NS(((Uint64)time_hi << 32) | (Uint64)time_lo);
 
-    dx_unaccel = wl_fixed_to_double(dx_unaccel_w);
-    dy_unaccel = wl_fixed_to_double(dy_unaccel_w);
+    /* If the mouse scale function pointer is the system acceleration function,
+     * just return the accelerated data here. The transform function is a no-op.
+     */
+    if (SDL_GetMouse()->ApplySystemScale != Wayland_ApplySystemScale) {
+        dx = wl_fixed_to_double(dx_unaccel_w);
+        dy = wl_fixed_to_double(dy_unaccel_w);
+    } else {
+        dx = wl_fixed_to_double(dx_w);
+        dy = wl_fixed_to_double(dy_w);
+    }
 
     if (input->pointer_focus && d->relative_mouse_mode) {
-        SDL_SendMouseMotion(Wayland_GetEventTimestamp(timestamp), window->sdlwindow, input->pointer_id, true, (float)dx_unaccel, (float)dy_unaccel);
+        SDL_SendMouseMotion(Wayland_GetEventTimestamp(timestamp), window->sdlwindow, input->pointer_id, true, (float)dx, (float)dy);
     }
 }
 

src/video/wayland/SDL_waylandmouse.c

@@ -903,6 +903,11 @@ static SDL_MouseButtonFlags SDLCALL Wayland_GetGlobalMouseState(float *x, float
     return result;
 }
 
+void Wayland_ApplySystemScale(void *internal, Uint64 timestamp, SDL_Window *window, SDL_MouseID mouseID, float *x, float *y)
+{
+    // Wayland sends both unaccelerated and accelerated relative motion data, so this is just a flag/passthrough function.
+}
+
 #if 0  // TODO RECONNECT: See waylandvideo.c for more information!
 static void Wayland_RecreateCursor(SDL_Cursor *cursor, SDL_VideoData *vdata)
 {
@@ -973,6 +978,7 @@ void Wayland_InitMouse(void)
     mouse->WarpMouseGlobal = Wayland_WarpMouseGlobal;
     mouse->SetRelativeMouseMode = Wayland_SetRelativeMouseMode;
     mouse->GetGlobalMouseState = Wayland_GetGlobalMouseState;
+    mouse->ApplySystemScale = Wayland_ApplySystemScale;
 
     SDL_HitTestResult r = SDL_HITTEST_NORMAL;
     while (r <= SDL_HITTEST_RESIZE_LEFT) {

src/video/wayland/SDL_waylandmouse.h

@@ -27,6 +27,7 @@
 extern void Wayland_InitMouse(void);
 extern void Wayland_FiniMouse(SDL_VideoData *data);
 extern void Wayland_SetHitTestCursor(SDL_HitTestResult rc);
+extern void Wayland_ApplySystemScale(void *internal, Uint64 timestamp, SDL_Window *window, SDL_MouseID mouseID, float *x, float *y);
 #if 0  // TODO RECONNECT: See waylandvideo.c for more information!
 extern void Wayland_RecreateCursors(void);
 #endif // 0