- more flexible linearization of depth, doesn't impact performance
at all in the shader, but takes into account 3 parameters of
the projection matrix instead of true. Mathematically identical
to before.
- use depth test "less or equal" for depth only passes to be consistant
with the color pass. Unsure if using "less" would be better.
- use textureLod() when sampling the skybox
- use mediump for all samplers in the shadowing.fs, which correspond to
their actual definition.
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