[3.x] Add soft shadows to the CPU lightmapper

[JFonS]

Adds the light_size property to Lights. It's only considered in baked lightmaps for soft shadowing purposes.

Based on @RandomShaper's work in JFonS#3

[lawnjelly]

Did you think about making light_size a Vector3? That way maybe you could have e.g. long thin lights and rotate them with the transform. Of course you can use emission to have more control over a light volume but a Vector3 is quite handy.

One thing I remember is if you place a volumetric light by a wall, you can end up getting light cast on objects from the other side of the wall, it can be kind of annoying in a confined space like a room. I think I solved it by doing a ray check from the centre of the light to the light start position before doing the main ray trace to the surface. If there is no path from the centre to the light start, the ray fails.

[JFonS]

@lawnjelly sorry, I missed this somehow.

I would keep the light_size property as a float because it will have forward compatibility with master, also most people just want to soften shadows, with a Vector3, they would have to set the 3 components to the same value, so it less usable for the common case.

Light leaking is most likely a problem here too (I haven't tested), but I'm a bit swamped with things to do right now. I think we can merge the PR as it is and add some optional checks if we see it reported often.

[akien-mga]

Thanks!