MythZoneMinder: fix playback of continuous events

Not every frame is stored in the database for continuous events so their
playback was broken. This fixes the problem by reading the number of frames
and the event time from the DB and interpolating the frame list and time deltas
from that.

Refs #8912

(cherry picked from commit 68f98331c2e2053e803d6237eacb75a7294c2825)
MythZoneMinder: get a normal frame image if an analysis image is not available

Continuous (record) events don't produce any alarm frames so no analysis images
are produced.  It is also possible to turn analysis image creation off even for
alarm events so fall back to returning a normal frame image if no analysis
image is found.

Fixes #8912

(cherry picked from commit f7d05529b1980638729f2e232fc46c346fe78a0e)