Wayland Backend: fix crash due to steam notifications

[sharkautarch]

Fixes issue: #1690

[dyfrgi]

If I understand this patch correctly, what it's doing is saying that, if the plane we're trying to present has source X or source Y below zero, source width or source height very close to zero, or destwidth or destheight <= 0, don't present it.

But why are those being sent to begin with? And why does having moderately large output size lead to this? It seems like there is some bad math elsewhere which is causing the issue. I suspect that the plane containing the Steam notification meets the characteristics described above and that's why the notification is no longer visible.

Did you consider reusing ClipPlane?

Reading through WaylandBackend.cpp, there's a lot of math there which makes a lot of unstated and unchecked assumptions about the relationships between the sizes and offsets of source and target surfaces. I wonder if going through and adding some assert() statements akin to what you have here might bear some fruit in finding where the bad data is coming from.