win-dshow: Fix compiling with FFmpeg 7.0

[RytoEX]

Description

Use "new" (FFmpeg 5.1+) channel layout API as needed.

Motivation and Context

Want to be able to compile against FFmpeg 7.0.

Follow-up to:

• libobs: Fixes for the new FFmpeg channel API #7462

How Has This Been Tested?

Built locally on Windows 11. I'm not clear on how to activate this code path, so further review would be appreciated.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[pkviet]

The fix is correct.
Sidenote left here for the sake of documenting it:
However the original code is not ideal because it assumes a given number of channel is enough to set the layout.
For instance, if the source has octogonal audio (8.0), it should be resampled first to 7.1 since obs only knows 7.1.
Another example is the various 5.1 arrangements (5.1 side, 5.1 wideback, 5.1 back ...) which will be treated as identical to our 5.1.
So the right call would be to call a swresampler instead of assuming the layout is ours.
BUT in practice this is exceedingly rare; we use the most common layouts so let's keep the code simple.