Cannot read subsequent frames

[fdoumet]

As HEIF supports burst mode, it's common for HEIF files to have multiple frames in the same file.
Reading a file with pyheif.read_heif() only reads the first frame.

Support needs to be added for reading subsequent frames.

[ghost]

I don't really have this use case but I can look at adding it if libheif supports it. Providing a sample file would be helpful.

[ax-at-work]

Sample files are provided by nokiatech at the https://github.com/nokiatech/heif_conformance repo.

[li-js]

Hope it can be supported ❤️

[ant32bit-carsales]

A WIP pull request has been made regarding this. If you feel like it, please review. #71