Add Texture.subTexture.

A super handy API (shamelessly borrowed from PlayN) that makes it really
natural to work with sub-regions of a texture atlas.

    var texture = pack.getTexture("Atlas").subTexture(100, 0, 50, 50);
    trace(texture.width == 50); // true

    // Shows only the 50x50 subtexture
    var image = new ImageSprite(texture);

    // Repeats only the subtexture region
    var pattern = new PatternSprite(texture, 100, 100);
Remove BitmapSprite, use subTexture to draw the region instead.

If you were relying on one of the removed BitmapSymbol properties,
switch to using the properties on the SubTexture. eg: bitmap.x is now
bitmap.texture.x, bitmap.atlas is bitmap.texture.parent, etc.