Xpra: Ticket #2773: pixel format conversion can restride

See r26433.

When using an xterm with --pixel-depth=30 and a client with a 24 bit display (that's over 99% of them), we convert r210 to BGRA and only restride afterwards. So we end up copying huge amounts of data, only to discard it on the next step.

Sun, 05 Jul 2020 14:03:48 GMT - Antoine Martin: status changed; resolution set

Done in r26891.

Sat, 23 Jan 2021 06:00:25 GMT - migration script:

this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2773