Import libliftoff as subproject and plumb our planes through it.
Wait for a flip to complete immediately after queuing it for now.
Also immediately wait for cursor image uploads, as we don't have to
do Vulkan rendering anymore after queuing an upload.
Cursor shows up with format hack, but is badly corrupt. We might need
to use a DRM dumb_fb rather than going through the same Vulkan image.
Alpha bits of surfaces are ignored again, need to make Steam select
an ARGB visual for overlay windows, probably.
Setting up the "alpha" plane property somehow makes overlays not
show up at all despite liftoff happily letting us go with scanout.
