video: wake up core as soon as possible

There's no reason to let the core wait until the frame is done
displaying. In practice, the core normally didn't need this additional
wakeup, and the VO was quick enough to fetch the new frame, before the
core even attempted to queue a new frame. But it wasn't entirely clean,
and the correct wakeup handling might matter in some cases.
TOOLS/mpv_identify.sh: simplify line-by-line reading

Use here-document idiom to read mpv output line-by-line.
TOOLS: eliminate echoes with variable substitutions

echo behaviour with backslash escapes seems to be non-portable: dash does
expand such an escapes and bash does not, so use cat/printf instead.