gpu,gpu/gl: specialize Buffer.Bind to Buffer.BindIndex

Direct3D can't support a generic Bind, and we don't need it now
BindVertex was added.

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

gpu/backend.go

@@ -116,7 +116,7 @@ type Uniform interface{}
 
 type Buffer interface {
 	BindVertex(stride, offset int)
-	Bind()
+	BindIndex()
 	Release()
 }