Avoid unnecessary glBindVertexArray() calls

It is called before and after every draw call to bind and unbind.
Now it is called when the VAO to be bound is different from previous draw call
and only unbound at the end of View3D render layer.

Change-Id: I9372e59269c5248aece66834903dec5d01b2ef38
Reviewed-by: Antti Määttä <antti.maatta@qt.io>
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>