virgl: work around bad assumptions in virglrenderer

Virglrenderer does the wrong thing when given an instance divisor;
it tries to use the element-index rather than the binding-index as
the argument to glVertexBindingDivisor(). This worked fine as long
as there was a 1:1 relationship between elements and bindings,
which was the case util 19a91841c34 "st/mesa: Use Array._DrawVAO in
st_atom_array.c.".

So let's detect instance divisors, and restore a 1:1 relationship in
that case. This will make old versions of virglrenderer behave
correctly. For newer versions, we can consider making a better
interface, where the instance divisor isn't specified per element,
but rather per binding. But let's save that for another day.

Signed-off-by: Erik Faye-Lund <[email protected]>
Fixes: 19a91841c34 "st/mesa: Use Array._DrawVAO in st_atom_array.c."
Reviewed-by: Mathias Fröhlich <[email protected]>
Tested-By: Gert Wollny <[email protected]>

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