vrend: check negative level in transfer info validation

Ryan Neph requested to merge ryanneph/virglrenderer:fix-level into master

GL API doesn't allow negative level, despite defining as GLint. Vrend only expects >= 0, even when minigbm's virgl backend sometimes re-purposes the "level" field of protocol messages to guest bo stride.

Signed-off-by: Ryan Neph ryanneph@google.com

/cc @afrantzis @gerddie

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