skins2: fix minor artefact with animated image

Don't do anything if the image doesn't change. Otherwise, some unpleasant
flickering occurs when the image is animated.

modules/gui/skins2/controls/ctrl_button.cpp

@@ -132,6 +132,9 @@ void CtrlButton::draw( OSGraphics &rImage, int xDest, int yDest, int w, int h )
 
 void CtrlButton::setImage( AnimBitmap *pImg )
 {
+    if( pImg == m_pImg )
+        return;
+
     AnimBitmap *pOldImg = m_pImg;
     m_pImg = pImg;
 

modules/gui/skins2/controls/ctrl_checkbox.cpp

@@ -159,6 +159,9 @@ void CtrlCheckbox::draw( OSGraphics &rImage, int xDest, int yDest, int w, int h
 
 void CtrlCheckbox::setImage( AnimBitmap *pImg )
 {
+    if( pImg == m_pImgCurrent )
+        return;
+
     AnimBitmap *pOldImg = m_pImgCurrent;
     m_pImgCurrent = pImg;