Add tile mode support for RoundedImageView

Addresses #66.
I'm mostly going off of how `BitmapDrawable` handles this issue, but I see logic in rebuilding during `draw()`. Otherwise, if you set both x/y tilemode at the same time, it'll have to rebuild it twice in a row. Plus, it helps for the matrix issue (if you're tiling, you *don't* want a scaling local matrix on the shader, or else the tiling will just expand to fill the view).