Add BufRead + Seek bound on many decoders

[fintelia]

The purpose of this change is to allow future decoder improvements:

• Experiments with the PNG crate have shown that working with a BufRead implementation rather than just a Read implementation can be faster.
• The Seek bound is useful for decreasing the memory usage of the decoder. Instead of storing all image metadata as it is seen, a decoder can record the location in the file of specific chunks and then seek to them if/when they are queried.

At the same time, not all decoders are likely to need either of these and having looser trait bounds is nicer for users. Thus I left the existing Read bound for some decoders (and actually removed the Seek bound on TGA). After this change, the trait bounds are: