Use linear filtering for painting height

[Smorty10]

Description

When painting the world and using a very large brush size like everything over 150 meters, the aliasing of the brushes become painfully obvious. This can be seen in this image

I used one of the basic round brushes to create this cliff looking terrain and due to the large brush size, the falloff becomes very step-like. Having the option to paint using a linearly sampled brush texture would get rid of this issue, as such step-like patterns would become interpolated and thus smoothed over.

The image is an extreme case of the problem, but it is still very obvious even with less strong brush stregnth as seen here I used the very smooth round brush for making this image with a brush size of 500 meters.

I would also advise giving the user the option to switch between nearest-neighbour (the current sampling method) and linear sampling in the brush settings list (at the bottom of the 3d viewport).

Notes

• I am aware that the smoothing tool can also get rid of this issue, but having the option to immediately paint with smooth terrain makes the editing process a lot quicker.