Revert "Move calculation of normalized position for the vignette into the vertex...

Revert "Move calculation of normalized position for the vignette into the vertex shader."

This is not correct, since tc can be changed by the fragment shader
_and_ by later vertex shaders. We need to do some rethinking here,
seemingly.

This reverts commit 2b4a7d868112b4995bf6699f5b3aa68e9ec2b308.