FS-7824 Add functionality for capturing screenshots from both legs to uuid_write_png

FS-7824 Add functionality for capturing screenshots from both legs to uuid_write_png

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.