tkhd should parse track height and width as fixed point numbers #178

the tkhd parser currently parses the width and height fields of a track as u32.

According to the MPEG4 spec, these fields are fixed-points.

This leads to the parsed values being off by a factor of 65536.

It looks like this is handled in the C API by shifting the number right 16 bits

Unfortunately, Rust doesn't provide built-in fixed-point support but it looks like a couple of crates implement it (see fixed), but this would require adding a dependency. I'm also not sure if that crate provides an interface for parsing fixed-point numbers from binary.