FlxTilemap#loadMap(): support Array<Array<Int>> instead of Array<Int>, closes #1290

Supporting both 2D and 1D-arrays here is suprisingly difficult due to the use of `Dynamic` and target-specfic differences, so I decided to drop support for the latter. A 2D array is much more natural representation of map data anyway. Loading maps with array is very inconvenient / awkward as is, as `widthInTiles` and `heightInTiles` have to be set in advance.
why casting here?