Terrain: Implement a better cursor

[xoxota99]

So far, we have tried two different cursor methods:

• The "Cone". This is okay. Bottom of the cone didn't really meet the ground when it was rough, and the cone was hard to see through (translucency didn't really help). The cone was also subject to any lighting attached to the root node (though to be honest, I probably could have fixed that with materials).
• The "Spotlight". This was visually interesting, but unintuitive to the user, and behaved strangely when focused on, say, the top area of a mountain. The spotlight would throw light all the way to the bottom of the mountain, making the area of effect look bigger than it actually was. Pretty, but not very functional.
• The "Blob". This is up next. One of the DiffusionMaps in the Terrain material will be dedicated to displaying a "Spotlight"-like Blob, with Radius and Intensity. This should be more like what we see in, say, SimCity. Initial Blob will not be animated or pulsing or anything special.

[xoxota99]

Blob Cursor is working, but isn't optimized. Think about setting the Cursor AlphaChannel to always be "full on", and instead update the DiffuseMap with a (semi-) generated texture. If the Cursor has changed position, but not radius or power, can we save some cycles by "moving" the DiffuseMap Image, instead of re-generating it?