When calculating the linear depth for VSM, we were using the whole
range between 0 and the far plane, the near plane wasn't taken into
account.
This can be a problem is the light is very far, but it's near plane is
closer to the camera/scene, in this case the depth precision wasn't
used optimally.
Note that we don't hit this problem currently, because the directional
light is constructed such that its origin is at the near plane, and the
spotlights have a fixed near plane (which is a problem and will be
fixed at a later time).
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