stbt.Display.frames: Document format of yielded images
stbt.Display.frames: Disable timeout by default

Note that the public API to this function is via "detect_match",
"wait_for_match", "detect_motion", etc. Those functions all continue to
take a default "timeout_secs" of 10, which they pass on to "frames". So
existing users won't be affected. New users will be able to use frames
to get an unlimited supply of frames, if they so desire. (At least,
they *will* be able to once we make "frames" publicly accessible.)

Note however that the "no input video" timeout of 10 seconds will still
apply.

The unit test takes 12s to run, because the previous default timeout was
10s. Hopefully this won't increase the total runtime of the test suite
too much, at least if you install GNU parallel (see 42d1d8aa).