riff: Provide correct media type for MSS1 and MSS2

Windows Media Video Screen (WMV Screen) are video formats that
specilise in screencast content. This provides a correct media type
for them instead of just video/x-asf-unknown.
pbutils: add description for MSS1 and MSS2 windows media formats
Require orc >= 0.4.18

Which contains important bug-fixes.

https://bugzilla.gnome.org/show_bug.cgi?id=698520
videoencoder: Check for invalid stop position before calculating a duration from it

https://bugzilla.gnome.org/show_bug.cgi?id=707332
videoconvert: improve debug
videoconvert: fix handling of chroma resample

Increase the number of temporary lines that we need, it is possible that the
up and downsampling offsets are out of phase and that we need to keep some
extra lines around. Also copy the unhandled output lines for the next round
instead of overwriting them.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=706823
videoconvert: only chroma subsample when needed
po: update POTFILES.in

https://bugzilla.gnome.org/show_bug.cgi?id=707158
typefind: Added "dash" and "avc3" fourCC codes to qt_type_find.

This commit adds detection of the "dash" and "avc3" compatible brands
in qt_type_find.

Amendment 2 of ISO/IEC 14496-15 (AVC file format) is defining a new
structure for fragmented MP4 called "avc3". The principal difference
between AVC1 and AVC3 is the location of the codec initialisation
data (e.g. SPS, PPS). In AVC1 this data is placed in the initial MOOV
box (moov.trak.mdia.minf.stbl.stsd.avc1) but in AVC3 this data goes in
the first sample of every fragment (i.e. the first sample in each mdat
box).  The principal reason for avc3 is to make it easier for client
implementations, because it removes the requirement to insert the
SPS+PPS in to the decoder pipeline every time there is a representation
change.

https://bugzilla.gnome.org/show_bug.cgi?id=702004
videoconvert: add more fastpaths

Also reuse the I420 code for YV12 because it can handle the swapped UV fields
just fine.
Update specfile with latest changes
videoconvert: also allocate temp lines in fastpath

Some of the fastpath functions need tmplines, so make sure we allocate some in
the fastpath too.

This avoids SEGFAULTs with odd heights.

See https://bugzilla.gnome.org/show_bug.cgi?id=663248
videoconvert: fix height round down
videoconvert: handle lines in one go

Handle odd heights in 1 go when no vertical subsampling is used.
video-format: don't overrun the arrays on UYVP
video-format: handle odd width in various pack functions
video-format: clean up pack/unpack functions
video-format: fix unpack functions for odd formats
videoconvert: don't convert too much with odd width
video-format: fix chroma offsets