IconButtons don't fill up the space they're given

[Aran-Fey]

To reproduce:

rio.Container(
    rio.IconButton('material/castle', size=0),
    width=5,
    height=5,
)

The expected result would be an IconButton with size 5x5, but the actual result is no button at all.

[Aran-Fey]

When this is fixed, we should also allow setting the size to 'grow'.