code/h264/decode: use BitReader for parsing function calls.

[Saxon Milton]

We currently have calls to parsing functions like readUe:

https://bitbucket.org/ausocean/av/src/6880cc329833cc0ba471632c0a0adcdc6f77da0f/codec/h264/decode/slice.go#lines-376

that need to be given a BitReader, not nil. We need to find all calls to parsing functions defined in parse.go and give them a BitReader.

[Saxon Milton]

Addressed in PR #214