cacti no longer spawn outside of desert variants

Cacti have an additional check in cFinishGenSprinkleFoliage to make sure they are in a desert biome variant.

resolves #1629
resolves #1632 
The comment is somewhat misleading, especially for people with worse English capability. How about 'Returns true if the specified biome is a desert or its variant`? And make the function static, it doesn't require an instance of the object.