gsk: Reuse VAOs with identical buffers

gsk: Reuse VAOs with identical buffers

Just like we reuse texture ids with the same size we can, at the expense
of a little memory, reuse vertex buffers if they reference the same
attributes and contain the same data.

Each VAO is marked as free at the end of the frame, and if it's not
reused in the following frame, it gets dropped.