vbo: Avoid unnecessary copy to/from current in vertex format upgrade.

Rebuilding the vertex format from scratch every time we see a new
vertex attribute is rather costly, new attributes can be appended at
the end avoiding a copy to current and then back again, and the full
attr pointer recalculation.

In the not so likely case of an already existing attribute having its
size increased the old behavior is preserved, this could be optimized
more, not sure if it's worth it.

It's a modest improvement in FlightGear (that game punishes the VBO
module pretty hard in general, framerate goes from some 46 FPS to 50
FPS with the nouveau classic driver).

Signed-off-by: Brian Paul <brianp@vmware.com>

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