Add "DefaultServerLayout" option

This fixes issues where keyboard and mouse inputs were not recognised if an /etc/X11/xorg.conf.d/ config overrides the intended "ServerLayout" section.

This patch is based on a recent upstream commit, which will be released with the package v0.2.16 around April, which is likely too late to be fully merged into Debian Bullseye, hence this patch is provided. It is considered important enough since most Raspbian/Raspberry Pi OS users as well as other SBC users have an xorg.conf override to disable screen blanking.

Related upstream issues:

• https://github.com/neutrinolabs/xorgxrdp/issues/164
• https://github.com/neutrinolabs/xrdp/issues/1701
• https://github.com/neutrinolabs/xrdp/issues/1779
• https://github.com/neutrinolabs/xrdp/issues/1784

I'm new to committing patches, hence bear with me and guide me how to format it correctly, in case :).