gpu,gpu/backend: [compute] handle loss of buffer contents during download

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/backend/backend.go

@@ -3,6 +3,7 @@
 package backend
 
 import (
+	"errors"
 	"image"
 	"time"
 )
@@ -225,6 +226,8 @@ const (
 	BlendFactorDstColor
 )
 
+var ErrContentLost = errors.New("buffer content lost")
+
 func (f Features) Has(feats Features) bool {
 	return f&feats == feats
 }