With this patch, uuid_write_png is able to capture screenshots from both
legs of a bridged video call. It still expects a uuid and filename for
the first two arguments. If given a third argument, it must be concat
or split. If concat is specified, screenshots from both ends are taken
and written side by side to the given filename. If one image is shorter
than the other then it is padded with black and vertically centered. If
the third argument is split then the two screenshots are written to
separate files. In this case, a second filename is expected as a fourth
argument.
This adds the API command uuid_write_png to mod_png which takes a uuid and
file as input. This command takes a screenshot from the session associated
with the given uuid and writes it to the given file. If an absolute path
is not specified, the images directory is used as a prefix.
