MSAA is a optimiztion to execute the fragment shader just once per pixel instead per sample.
It sounds great, but has a big issue: At edges where the center isn't in the polygon, the
fragment would still be executed, but still with the center of the pixel as position.
So if some calculations aren't allowed outside the polygon, the result would be invalid.
But the nice one: we can give a hint to each input to be choosen from a valid pixel,
so now every pixel will be calculated with valid source.
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