RDP: Fix H264 acceleration for connections to Windows hosts
On my system I was not able to connect to Windows hosts when setting acceleration to "Use Best Available". The issue seems to be caused by the color depth. For any accelerated mode, 24-bit color depth seems to be necessary. Remmina had similar issues and uses a RGB24 framebuffer for this reason: https://gitlab.com/Remmina/Remmina/-/issues/2456
This MR changes two things:
- select 24 bit color depth when any acceleration is requested
- change the framebuffer from RGBA8888 to RGB24 to allow H264 acceleration
@akallabeth, can you maybe comment if the changes are feasible, as you have also been involved in the Remmina issue.