Fix color problems with overlays (fix #1914)

There were two problems:
1) Overlays weren't using the screen color space, so restoring the
   pixels were modifying the original saved area
2) A custom cursor (when "Use native cursors" option were enabled) was
   using overlays, when we could use a native custom cursor
   anyway (without overlays)

src/ui/overlay.cpp

@@ -78,8 +78,12 @@ void Overlay::captureOverlappedArea(os::Surface* screen)
   if (!m_surface)
     return;
 
-  if (!m_overlap)
-    m_overlap = os::instance()->createSurface(m_surface->width(), m_surface->height());
+  if (!m_overlap) {
+    // Use the same color space for the overlay as in the screen
+    m_overlap = os::instance()->createSurface(m_surface->width(),
+                                              m_surface->height(),
+                                              screen->colorSpace());
+  }
 
   os::SurfaceLock lock(m_overlap);
   screen->blitTo(m_overlap, m_pos.x, m_pos.y, 0, 0,

src/ui/system.cpp

@@ -82,8 +82,7 @@ static bool update_custom_native_cursor(const Cursor* cursor)
   bool result = false;
 
   // Check if we can use a custom native mouse in this platform
-  if (!use_native_mouse_cursor &&
-      support_native_custom_cursor &&
+  if (support_native_custom_cursor &&
       mouse_display) {
     if (cursor) {
       result = mouse_display->setNativeMouseCursor(
@@ -167,7 +166,8 @@ static void update_mouse_cursor()
   }
 
   // Try to use a custom native cursor if it's possible
-  if (!update_custom_native_cursor(cursor)) {
+  if (nativeCursor == os::kNoCursor &&
+      !update_custom_native_cursor(cursor)) {
     // Or an overlay as last resource
     update_mouse_overlay(cursor);
   }