Add back KMS cursor hotspot
Quoting the commit message of the second last commit:
The transactional KMS API has been modelled after atomic KMS. Atomic KMS currently doesn't support forwarding cursor hotspot metadata, thus it was left out of the transactional KMS API having the user set the simply create a plane assigment with the cursor sprite assigned to a cursor plane using regular coordinates.
This, however, proved to be inadequate for virtual machines using "seamless mouse mode" where they rely on the cursor position to correspond to the actual cursor position of the virtual machine, not the cursor plane. In effect, this caused cursor positions to look "shifted".
Fix this by adding back the hotspot metadata, right now as a optional field to the plane assignment. In the legacy KMS implementation, this is translated into drmModeSetCursor2() just as before, while still falling back to drmModeSetCursor() with the plane coordinates, if either there was no hotspot set, or if drmModeSetCursor2() failed.
Eventually, the atomic KMS API will learn about hotspots, but when adding our own atomic KMS backend to the transacitonal KMS API, we must until then still fall back to legacy KMS for virtual machines.