Prevent division by 0 when creating vertices of a PrismMesh

[LeoBelda]

Fixes #86356

When creating a PrismMesh with a height value of 0, a division by 0 occurs in PrismMesh::_create_mesh_array, leading to nan values in vertices, and then to undefined behavior when using the resulting mesh array.

First approach was to simply protect the divisions this way:
float scale = size.y != 0 ? (y - start_pos.y) / size.y : 0
It fixed the crash by preventing nan values to arise, but led to incorrect x coordinates for some of the vertices:

This is because we are relying on the current ratio of the y coordinate to compute scale.

Now if we focus on the y variable, we see it is initialized before the loop
y = start_pos.y;
and incremented at the end of the loop for every iteration
y += size.y / (subdivide_h + 1.0);
So we can affirm that at the beginning of the loop, y == start_pos.y + j * (size.y / (subdivide_h + 1.0))

Now back to the scale calculation:
float scale = (y - start_pos.y) / size.y
Let's replace y with the expression we just established and simplify it:

scale = (start_pos.y + j * (size.y / (subdivide_h + 1.0)) - start_pos.y) / size.y
scale = j * (size.y / (subdivide_h + 1.0)) / size.y
scale = j / (subdivide_h + 1.0)

Notice how we got rid of the division that was causing the issue.
Also notice that the variable v2 was already assigned with this operation; I kept it as an alias of scale to keep naming consistent, not sure it is the best choice.

With this solution we have consistent coordinates even when the prism is flat (size.y == 0) :

[clayjohn]

Looks great! Tested locally and can confirm that it works.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[YuriSizov]

Cherry-picked for 4.2.2.