kmsro: look for graphics capable screen as renderonly device
Right now renderonly assumes that all rendernodes provide full 3D graphics rendering capabilities. This isn't necessarily true, as a rendernode might also expose compute-only or 2D graphics capabilities. This leads to false matching of render devices to KMS devices when there is more than a single rendernode in the system and they all match the supported driver list in kmsro. Instead of using the first rendernode we find, iterate the rendernodes until we find one where a graphics capable screen can be constructed.
Draft for now, as I don't really like the aesthetics of the iteration cursor being plumbed through the loader calls and dri2 queryCompatibleRenderOnlyDeviceFd
still missing the same treatment.
CC @tomeu