This will be used later in both the mpeg2_metadata filter and the
VAAPI MPEG-2 encoder. Also adds a unit test.
(cherry picked from commit b5859e0b04bdbe12c97cb12ac10a45d51d2d73c9)
* commit 'fe27792fd779ac4cdd5e57be5f6f488483c307b2':
build: Move ff_mpeg12_frame_rate_tab to a separate file
Merged-by: Hendrik Leppkes <h.leppkes@gmail.com>