vispy / vispy / 20271400811

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Fix interaction between instance colors and face colors (#2705)

I realized that somehow our `examples/scene/instanced_mesh_visual.py`
example is broken since the beginning. Digging a bit deeper, I found out
that the overlap between face colors, mesh color and instance colors was
causing some mismatch between buffers and all sorts of strange
behaviours. Try out the example and press a few times the relevant
shortcuts (`f`, `c`, and `t`). On main you'll see crazy stuff that now
should look reasonable :P

One difference in the *intended* behaviour is that now the instance
color and the underlying mesh color are mixed multiplicatively. This is
the same as how the texture filter behaves with face and mesh colors, so
I figured it would make sense to follow the same pattern.

Before:

<img width="802" height="602" alt="image"
src="https://github.com/user-attachments/assets/91359921-f254-41b9-8aa8-135df5ff5725"
/>


After:

<img width="802" height="602" alt="image"
src="https://github.com/user-attachments/assets/2f7fdccd-5a9c-4717-9ccb-310c6d2f19eb"
/>

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