Rendering Engine: Fix blending when blending parameters per target are not supported

[Popov72]

See https://forum.babylonjs.com/t/possible-incompatibility-between-order-independent-transparency-and-transparent-gridmaterial/60163/12

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[sebavan]

Is there a way to prevent adding more conditions in alphaState ? it sounds like separate blending is adding cost to the simple blend which is used 99.9% of the time. I wonder how we could support both without extra perf impact

[Popov72]

Is there a way to prevent adding more conditions in alphaState ? it sounds like separate blending is adding cost to the simple blend which is used 99.9% of the time. I wonder how we could support both without extra perf impact

I refactored the code to have a single additional check.

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