[mythtv] ivtv support classes and deviceIsMpegFile

[Unit3]

I've just been poking around in mpegrecorder.cpp, which I assume is used
to support the hardware MPEG encoding cards available. However, I'm more
interested by the presence of the deviceIsMpegFile flag. It looks like
this is used to specify a static file you can use as a capture source
instead of one of these files. Would it make sense to extend this
slightly, so that this module could also take MPEG as output from any
arbitrary application?

I'm thinking of something along the lines of passing it
"app:udpmulticastgrabber.pl channel=12", and then in
MpegRecorder::Open(void) it just calls a function like
OpenMpegAppAsInput(), which takes the standard output of whatever app
you've specified and uses that as your mpeg input.

This would prevent there being any hard dependencies on any external
apps like VLC / netcat / whatever, make it more flexible for really odd
configurations (as I'm sure there's lots of things you could get an MPEG
stream out of!), and, more importantly, be a lot less work than writing
a whole new udp-multicast specific capture source.

Any thoughts on this? I'm just sort of coming up with this off the top
of my head, so I'd welcome feedback on this before I actually start
hacking it together.

Graeme