add window capture support

[adryzz]

Closes #41

• x11grab window capture
• gdigrab window capture

[adryzz]

gdigrab works, although i've encountered a weird issue on the decoder thread where the calculation for how much time to sleep divides by 0 (e.g. decoder.Framerate.num is equal to 0)

[adryzz]

Apparently, the decoder frame rate is just a guess from FFmpeg, so when it's 0 we cam just fall back to a default value, as the worst that's gonna happen is a little higher CPU usage

[adryzz]

Apparently gdigrab cannot grab UWP windows...

https://trac.ffmpeg.org/ticket/7718

[adryzz]

i did not forget about this PR, finally gotten around to submitting the ffmpeg patch this depends on, we'll see how it goes from there

[adryzz]

WE ARE IN LETS GOOOOOOOOOOO

FFmpeg/FFmpeg@d7ac710

ill finally finish this pr lol

[adryzz]

welp it's definitely in mainline by now, time to update and stuff

[Speykious]

Seems like I got the time to make a complete rewrite PR before this one got updated. xD
I think it's best I close this one in favor of #51. Though I'm sure we can reuse all the interop code for GDI support.