x11: Enable the relative mouse system scale hint.

Use the scaled motion values if the relative system scale hint is set, and a custom transformation function is not set.

src/video/x11/SDL_x11xinput2.c

@@ -74,9 +74,13 @@ static bool xinput2_scrolling_supported;
 
 static void parse_relative_valuators(SDL_XInput2DeviceInfo *devinfo, const XIRawEvent *rawev)
 {
+    SDL_Mouse *mouse = SDL_GetMouse();
     double processed_coords[2] = { 0.0, 0.0 };
     int values_i = 0, found = 0;
 
+    // Use the raw values if a custom transform function is set, or the relative system scale hint is unset.
+    const bool use_raw_vals = mouse->InputTransform || !mouse->enable_relative_system_scale;
+
     for (int i = 0; i < rawev->valuators.mask_len * 8 && found < 2; ++i) {
         if (!XIMaskIsSet(rawev->valuators.mask, i)) {
             continue;
@@ -84,7 +88,7 @@ static void parse_relative_valuators(SDL_XInput2DeviceInfo *devinfo, const XIRaw
 
         for (int j = 0; j < 2; ++j) {
             if (devinfo->number[j] == i) {
-                double current_val = rawev->raw_values[values_i];
+                const double current_val = use_raw_vals ? rawev->raw_values[values_i] : rawev->valuators.values[values_i];
 
                 if (devinfo->relative[j]) {
                     processed_coords[j] = current_val;
@@ -102,7 +106,6 @@ static void parse_relative_valuators(SDL_XInput2DeviceInfo *devinfo, const XIRaw
     }
 
     // Relative mouse motion is delivered to the window with keyboard focus
-    SDL_Mouse *mouse = SDL_GetMouse();
     if (mouse->relative_mode && SDL_GetKeyboardFocus()) {
         SDL_SendMouseMotion(rawev->time, mouse->focus, (SDL_MouseID)rawev->sourceid, true, (float)processed_coords[0], (float)processed_coords[1]);
     }