Fix Python API screen buffer uses unblended palette

[lmBored]

As discussed on Discord, Python API screen buffer uses VIZ_CopyBuffer(), this calls screen->GetPalette() which returns SourcePalette. This is unblended base palette, so functions like FadeTo() is not visible and doesnt work.

[lmBored]

@mwydmuch Hi, I just want to ask if you maybe had a chance to take a look at this PR? :)

Thank you in advance.

[mwydmuch]

Hi @lmBored, thanks for bumping this PR. I was quite occupied with other things last two weeks, but I should take a look into this PR by the end of this week.

[lmBored]

Interesting, this test passed locally for me (I'm on Mac M series). The flash state timing is system-dependent ig, good that CI runner catches this

[Trenza1ore]

Interesting, this test passed locally for me (I'm on Mac M series). The flash state timing is system-dependent ig, good that CI runner catches this

Yeah M-series Mac just runs ViZDoom so well, many subtle bugs on Windows (and Ubuntu in this case) are hidden.

PS: I feel that this change needs to correspond to a new, separate ScreenFormat as this is 1) breaking change and 2) not strictly deterministic (possibly timing-based) as results shown.