mp_image: deal with FFmpeg PSEUDOPAL braindeath

We got a crash in libavutil when encoding with Y8 (GRAY8). The reason
was that libavutil was copying an Y8 image allocated by us, and expected
a palette. This is because GRAY8 is a PSEUDOPAL format. It's not clear
what PSEUDOPAL means, and it makes literally no sense at all. However,
it does expect a palette allocated for some formats that are not
paletted, and libavutil crashed when trying to access the non-existent
palette.