Fixed potential overflow in multiview vertex buffer validation

[m3x3]

Fixed a potential integer overflow in the "valid" path of multiview vertex buffer validation when max_instance_index equals u32::MAX. In this case, the addition max_instance_index + 1 on the right-hand side of the comparison overflows even for valid cases. The potential overflow in the error path (already marked "TODO") is not fixed by this.

I got hit by this issue when trying to run the multiview example on a Radeon 6700 XT with the amdgpu driver. The MESA driver apparently returns i32::MAX for max_instance_index and so it doesn't cause this overflow.

[Rua]

Thanks for catching this!