Allow inputs to say they cannot support mipmaps.

Allow inputs to say they cannot support mipmaps.

Really only FlatInput can easily support mipmaps; for things like YCbCrInput
that combine multiple inputs, it's hard (probably not downright impossible,
but at least not immediately obvious without thinking about it a bit) and for
FFTInput it makes no sense.

Thus, we allow an input to say that it can't do this, and then bounce it
to a texture if needed. Hopefully this should happen quite rarely.