Fix T60935: More numerically stable distance to ray computation

The old function was numerically very unstable for 2 reasons:
computing the square and then subtracting the results.

In the example in T60935 all precision was lost and it returned the distance 0
for all points.

I also removed the `depth` parameter since it wasn't used and computing
it would have made the code more complicated.

Reviewers: brecht, campbellbarton

Differential Revision: https://developer.blender.org/D4308

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