Mgfractal: Improve spawning behaviour

Mgfractal: Improve spawning behaviour

Spawn player 1 node higher to avoid spawning waist-deep in a possible
biome 'dust' node, such as tundra snowblock.
Tune default offset to spawn players in a more interesting location on the
mandelbrot sets, on a raised area that looks like a spawn platform.
Tune julia parameters to help avoid spawn search failing, especially for
fractal 6.