vfwcap: Add fallback define for HWND_MESSAGE

Some obsolete versions of the MinGW32 runtime (<4.0.0) lack the definition.
vc1: Fix the skip condition

As written in the comment above, skip must be added only if a
start code is found.
mpeg: Write H264 streams at offset 2

It hints that the content is H264 according to HD-DVD.
Merge commit 'ab56fabe6294524e99815451ad01e4ff50c6d734'

* commit 'ab56fabe6294524e99815451ad01e4ff50c6d734':
  vfwcap: Add fallback define for HWND_MESSAGE

The merged commit reverts the HWND_MESSAGE removial, and adds a #ifndef around
    commit 8bc52dbd9dffb1b2fa4a6aeed2d298d036b619b2
        vfwcap: Drop fallback VfW defines
        The defines were added long ago when MinGW still lacked them.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'a4d3c20035946cbc1509aec2dc28d51c2a2f9a8e'

* commit 'a4d3c20035946cbc1509aec2dc28d51c2a2f9a8e':
  vc1: Fix the skip condition

Conflicts:
	libavcodec/vc1_parser.c

See: ede411dd03950b91c6346f2a3ae0e7546555f42f
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '4e9e6fa99f3ff83cedbf6c378d62065ae419a3b9'

* commit '4e9e6fa99f3ff83cedbf6c378d62065ae419a3b9':
  mpeg: Write H264 streams at offset 2

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/snowenc: remove out-commented assert

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/nsvdec: fix out-commented asserts so the function names exist

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/utils: remove assert that tests the same condition as the if() directly above

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>