Wayland Weston blank screen with drm-backend

[John Frankish]

> Using mesa-18.0.0, Wayland-1.14 and Weston 3.0.0 I get a blank screen on a machine with intel
> hd4400 graphics when staring with:
> 
> WAYLAND_DEBUG=1 EGL_LOG_LEVEL=debug XDG_RUNTIME_DIR=/run/user/$(id -u) weston-launch >weston.log 2>&1
> 
> X works fine and, a couple of mesa upgrades ago, Weston worked fine - full output log attached,
> any troubleshooting suggestions would be appreciated.
> 
Ah - it seems it is due to using "--with-cairo=glesv2" rather than "--with-cairo=image", but why?