Added software::resampling::Context::get_out_samples to bind swr_get_out_samples

[liquidev]

When resampling, to make sure no samples are buffered internally, it's important to have a buffer large enough to fit all these samples. Up to this point there was no way of knowing how many samples should be allocated up front, so this PR adds a binding to that from libswresample.

Unresolved questions:

• swr_get_out_samples accepts an int, but I'm sure it expects the input sample count to be positive. It's probably an oversight on libswresample's part, so I kind of fixed it, and made the method return an error if the argument passed does not fit in an i32. It feels quite janky though so I need an opinion from the maintainers.