FFmpeg configuration updates fixing Windows build

[meepzh]

Linked issues

Summarize your change.

This builds upon the work that Mark did to improve the configuration options for FFmpeg in #360 with Mark's blessing!

Describe the reason for the change.

This fixes use of PKG_CONFIG_PATH, RV_FFMPEG_COMMON_CONFIG_OPTIONS, RV_FFMPEG_PATCH_COMMAND_STEP (#15 (comment)), and other Windows-specific build issues such as #67 (comment).

Describe what you have tested and on which operating system.

This has been tested on Arch Linux 6.6.8 and Windows 10.0.22631, running the same test from here.

Add a list of changes, and note any that might need special attention during the review.

Specific details can be found here. I did not include the change to apply SEPARATE_ARGUMENTS to RV_FFMPEG_CONFIG_OPTIONS, since RV_FFMPEG_CONFIG_OPTIONS did not seem applicable given the CMake documentation, but I can include that change as well.

If possible, provide screenshots.

#360 (comment)

Thanks!

[wbpa-garrett]

Chiming in to say that this patch fixed an openssl issue I was having with RV_DEPS_FFMPEG. Thanks for finding it!

[cedrik-fuoco-adsk]

Thanks for the contribution @meepzh!

I encountered the problem myself, and I came up with a similar solution. The difference is that I don't have that loop that you didn't like.

I'd like to propose a change for that loop. Do you have the time to do an extra commit? @meepzh
If you don't have time, no problem, I can take over the changes.

Once our internal CI runs on all platform, I'll comment about the refactor of the loop that I'd like to propose.

[meepzh]

Thanks Cedrik! Sure, I can add it. I saw the commit you've pinged me in and learned some new things about CMake! I'll await your comment then.

[cedrik-fuoco-adsk]

I was hoping to use cmake_path from CMake to convert the paths, but I'm not 100% confident that it works in some edge cases. So for now, your changes are great and fix the issues. Thank you!

I am currently testing your changes on our internal CI. @meepzh

[bernie-laberge]

This is fantastic @meepzh !
You fixed the issue I had on my end on Windows with Mark's original PR. Thanks!

[bernie-laberge]

Thank you so much for the fix !
The fix builds fine on my Mac+Windows and also on our internal CIs on all platforms.
This is great.