Return `unsafe.Pointer` instead of raw C type

[l0rem1psum]

As discussed in a previous PR #44, the raw C struct of AVFrame is returned instead of a unsafe.Pointer.

However, after some fiddling around, I finally discovered the difference of the two approaches.

Currently, the raw C struct returned will have a type of *astiav._Ctype_struct_AVFrame, and as the name suggests, this type is unexported. Hence, it comes with all the inconveniences of returning an unexported type.

To make it usable, one unavoidably has to do the following:

In conclusion, it's better to just return unsafe.Pointer and let the caller do the casting.

[asticode]

Thanks for the detailed explanation 👍 The best would be to replace UnsafeTypedPointer to UnsafePointer which would indeed return an unsafe.Pointer. Do you feel like creating a PR for this?