Allow user to set preferred format

Expose get_format to allow user to choose decoded pixel format, by adding a VideoCodecContext.preferred_format property. If set and supported by the decoder, that format will be used; otherwise fall back to current behaviour.

We could have exposed the callback directly for better control, but since there's codec.video_formats and PyAV tries to hide these complexities, I thought it was better this way. WDYT?

Handle format overrides, not set

The current code only updates VideoCodecContext.format and VideoCodecContext.pix_fmt when the user modifies them. However, according to the documentation, decoders may override those values too (at open, when decoding frames, etc.). The code now checks for pix_fmt / width / height changes at every access.

Also, when the pixel format is not set, pix_fmt now returns None instead of raising.