Plane, Sprite: Fix Bitmap flushes in draw handler

Comparing the current SceneElement classes to the earlier
written documentation actually immediatelly exposed some
bugs. Yay! :D

src/sprite.cpp

@@ -59,6 +59,8 @@ struct SpritePrivate
 
 	EtcTemps tmp;
 
+	sigc::connection prepareCon;
+
 	SpritePrivate()
 	    : bitmap(0),
 	      srcRect(&tmp.rect),
@@ -73,11 +75,15 @@ struct SpritePrivate
 
 	{
 		updateSrcRectCon();
+
+		prepareCon = shState->prepareDraw.connect
+		        (sigc::mem_fun(this, &SpritePrivate::prepare));
 	}
 
 	~SpritePrivate()
 	{
 		srcRectCon.disconnect();
+		prepareCon.disconnect();
 	}
 
 	void recomputeBushDepth()
@@ -112,6 +118,12 @@ struct SpritePrivate
 		srcRectCon = srcRect->valueChanged.connect
 				(sigc::mem_fun(this, &SpritePrivate::onSrcRectChange));
 	}
+
+	void prepare()
+	{
+		if (bitmap)
+			bitmap->flush();
+	}
 };
 
 Sprite::Sprite(Viewport *viewport)
@@ -313,8 +325,6 @@ void Sprite::draw()
 	if (emptyFlashFlag)
 		return;
 
-	p->bitmap->flush();
-
 	ShaderBase *base;
 
 	bool renderEffect = p->color->hasEffect() ||