Terrain Painting

LL recently released a teaser trailer for PBR + triplanar mapping on terrain: https://www.youtube.com/watch?v=Qmbws6tNwMw

However, the same old issues will presumably remain the same:

• You can only specify the low and high points for texture blending in each corner of the region.

• The viewer will then determine how the textures blend together.

• The blending is random, and done on the client side, so everyone sees a different terrain!

For those reasons, region owners have extremely limited control over their land, and that's why most people don't even bother using it, resorting to mesh instead. I have personally tried to make beaches in the past and found it absolutely impossible without setting the min blend heights very high, or setting all 4 textures to be the same.

Region owners need the ability to actually paint (like vertex painting) the terrain, in a similar way to the terraforming tools. 4 textures/materials is probably enough for most people, but more might be nice. This would allow the creation of designated themed spaces, beaches, paths, and more.

https://secondlife.canny.io/admin/board/feature-requests/p/terrain-painting

### Tasks
- [ ] https://github.com/secondlife/viewer/issues/1883
- [ ] #1884
- [ ] secondlife/server#1201