Embed autotile assets

[Geokureli]

Related to HaxeFlixel/flixel-demos#281

To Summarize: FlxGraphic.fromClass(GraphicAuto) results in a BitmapData of size 0x0 while it fetches the data asynchronously. When passed into a tilemap, no actual visible or solid tiles are created, this is why the EZPlatformer and SplitScreen demos don't work.

This change allows the autotile tilemaps to be used via "flixel/images/tile/autotiles.png". I plan to update the demos to do this if this is approved.

There may be other ways to fix this so people can use flixel's default autotile tilemaps. I don't know if there's some event that our demos could listen for before using the images, but I'd rather these demos avoid async listeners and look extremely simple to newcomers looking for snippets.

I don't know what effect this has on the export's file size to embed these images in every project. I assume they are already embedded via:@:keep @:bitmap. does this mean they are embedded twice? should we deprecate the @:bitmap classes?

[Geokureli]

export/html5/bin/ sizes (bytes):

Demo	old	new	diff
EZPlatformer	1,698,964	1,688,383	-10,581
SplitScreen	1,700,575	1,689,993	-10,582

/shrug/

[Gama11]

It seems like it should be possible to just make the existing GraphicAuto etc classes work. For instance, we could try turning it into a private class _GraphicAuto that is extended by the GraphicAuto class visible to the user. That subclass could then probably set width / height in its constructor.

[Geokureli]

I'll look into that, then. If we could avoid making people use arbitrary "flixel/images" paths than we should.