backend-rdp: report a zero physical size to compositor
The RDP-backend is reporting a non-zero physical size value, and there are some clients that get the resolution in pixels directly from the physical size reported. This leads to a resolution of 25.4 PPI (or 1px/1mm), which is too small.
But there's no need for that (as suggested by @emersion and @pq). The physical size is reported on enabling the output (in the case of RDP-backend we have no information about it before this), and the resolution is already set in this moment.
Report a zero physical size to compositor, what makes frontend and clients use their default values and applications become readable.
In #324 (closed), the application claws-mail
(running through RDP-backend and Xwayland) was used as example. Here we have screenshots of it before and after the fix: before_fix and after_fix.
Fixes: #324 (closed)