Log error if invalid Bitmap passed to FlxG.addBitmap()

[IQAndreas]

Resolves the following issue:

• FlxG.as line 738,743 undefined property bitmapData (FlxG.as line 738,743 undefined property bitmapData  #15, FlxG.as line 738,743 undefined property bitmapData  AdamAtomic/flixel#217)

[jquave]

I get an error from this commit:
/org/flixel/FlxG.as(754): col: 38 Error: Access of possibly undefined property ImgDefault through a reference with static type Class.

[IQAndreas]

Ah, my bad. I see what happened. It's a quick fix, so I'll get it up this weekend.

Normally I make sure to "test compile" the code before commiting it, but I guess I forgot this time. Thanks for the catch, @jquave!

[jquave]

I'm thinking we can just make ImgDefault public static, but not sure how that effects the existing use of ImgDefault. in FlxSprite.as.

[Dovyski]

I imagine the only reason why ImgDefault is not public is to avoid some sort of "class cluttering" for newcomers. I believe the change from private to public won't break anything and it's quite useful :)