Terrain Generator V0.8.1

Shortly after uploading version 0.8 I discovered and fixed a memory which was causing the performance to fall off a cliff after using it for a while, hence this update.

As well as fixing the memory leak it includes the following changes:

• In paint mode it's now possible to set the mask to only allow painting on areas which have been painted or not painted
• There's now a limit to how many operations will be saved in the undo buffer. It defaults to fifty, but can be changed in the settings panel

Note: for some reason, the first time the program is run after booting there can be a delay of up to 20 seconds between the unity logo disappearing and the loading screen appearing. Admittedly 20 seconds isn't a long time, but it can feel like a long time when you're looking at a black screen, wondering if the program has crashed.

technical details: I had wrongly assumed that any data that was sent to a compute shader would be garbage collected once the compute shader had finished. This turned out not to be the case causing the program's memory usage to grow 70MB every time the ApplyTextures compute shader was run. So now the memory space is assigned when the program first runs and then reused every time the ApplyTextures function is called.