new feature: added "tilesFromBitmap" to StarlingTileSystem

Since it can be useful to add bitmaps into the StarlingTileSystem (e.g.
when creating large images with ObjectMaker2D) I added the additional
method "tilesFromBitmap".

I've also moved most of the code from "tilesFromMovieclip" into the
second new method"tilesFromBitmapData" since it also used by
"tilesFromBitmap".

Finally I removed "pWidth" and "pHeight" since they are always equal to
"tileWidth" and "tileHeight" and removed "bitmap" and "rect" since their
content is used directly in the following line and then never again.