Two fixes, venomgfx Fraka bird test!

- AO "Use Distances" can not use 'sky texture' for colors, that bug was
  fixed in UI (prevent settings to combine), but better also gets fixed
  on reading files.

- Backbuf render, when not existed, didn't initialize backdrop color to 0.

source/blender/blenloader/intern/readfile.c

@@ -5398,11 +5398,13 @@ static void do_versions(FileData *fd, Library *lib, Main *main)
 		bArmature *arm;
 		bNodeTree *ntree;
 		
-		/* Migrate to Bullet for games, except for the NaN versions */
-		/* People can still explicitely choose for Sumo (after 2.42 is out) */
-		if(main->versionfile > 225) {
-			for (wo = main->world.first; wo; wo= wo->id.next)
+		for (wo = main->world.first; wo; wo= wo->id.next) {
+			/* Migrate to Bullet for games, except for the NaN versions */
+			/* People can still explicitely choose for Sumo (after 2.42 is out) */
+			if(main->versionfile > 225)
 				wo->physicsEngine = WOPHY_BULLET;
+			if(WO_AODIST == wo->aomode)
+				wo->aocolor= WO_AOPLAIN;
 		}
 		
 		/* updating layers still */

source/blender/render/intern/source/pixelshading.c

@@ -477,6 +477,11 @@ void renderSkyPixelFloat(float *collector, float x, float y, float *rco)
 
 static void fillBackgroundImage(float *collector, float fx, float fy)
 {
+	collector[0] = 0.0; 
+	collector[1] = 0.0; 
+	collector[2] = 0.0; 
+	collector[3] = 0.0; 
+	
 	if(R.backbuf) {
 		float dx= 1.0f/(float)R.winx;
 		float dy= 1.0f/(float)R.winy;