Fix: builtin forward simple shader

[Kushulain]

fixes #594

Move code which set the vertex color after the code which initialize vertex color to zero. So that vertex color is correctly set.

[CLAassistant]

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[atteneder]

That's a great find and solution!

I was able to reproduce the issue and the fix. It'll be merged internally and released in an upcoming release.

Thanks for your contribution ❤️

[atteneder]

Released in 6.6.0

Again, thanks!