Fix HTML5 CopyPixels

Using copyPixels were resulting in 

Uncaught TypeError: Cannot read property 'clearRect' of null ImageCanvasUtil.hx:102

Adding this line fix the problem

lime/graphics/Image.hx

@@ -252,6 +252,7 @@ class Image {
 			
 			case CANVAS:
 				
+				ImageCanvasUtil.convertToCanvas (this);
 				ImageCanvasUtil.copyPixels (this, sourceImage, sourceRect, destPoint, alphaImage, alphaPoint, mergeAlpha);
 			
 			case DATA:
@@ -1196,4 +1197,4 @@ enum ImageChannel {
 	BLUE;
 	ALPHA;
 	
-}
+}