Spotlights with shadows enabled cast light between mesh faces

[eon-s]

Godot version: Master f527a69

Issue description:
Spotlight with shadows (with default values) in some positions cast some light between mesh faces.

Steps to reproduce:

Add a spotlight with shadows enabled and put the center near a mesh face like this:

That line on the "floor" should not be there.

[akien-mga]

Would be good to have a minimal reproducer project to be able to test easily.

[eon-s]

Here you go (using ed6baff):
spotlightshadow.zip

Everything should look black instead of gray but that may be for another issue...

[eon-s]

I'm unable to reproduce this in a late january build.