test: add dumping of img_format metadata

This is fragile enough that it warrants getting "monitored".

This takes the commented test program code from img_format.c, makes it
output to a text file, and then compares it to a "ref" file stored in
git.

Originally, I wanted to do the comparison etc. in a shell or Python
script. But why not do it in C. So mpv calls /usr/bin/diff as a
sub-process now.

This test will start producing different output if FFmpeg adds new pixel
formats or pixel format flags, or if mpv adds new IMGFMT (either aliases
to FFmpeg formats or own formats). That is unavoidable, and requires
manual inspection of the results, and then updating the ref file.

The changes in the non-test code are to guarantee that the format ID
conversion functions only translate between valid IDs.

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