limit video helper meta data to the modes we will need

[Antoine Martin]

Typically, on Linux with X11 we will only need BGRX so there is no need to keep track of the conversions to and from BGR, RGB or RGBX, etc

[Antoine Martin]

Too difficult, most combinations are valid, they're just not very useful.

ie: when we have BGRX to h264 using x264 (or nvenc) then that's what we'll use, but going via YUV420P and a CSC step is also a valid option (which uses less bandwidth and CPU). We never want to use XRGB to h264, because that would require a BGRX to XGRB CSC step, but we don't know in advance that there are going to be better options..

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2226