Fix burnt vegetation hiding deep pits under ash piles

[chaosvolt]

Summary

SUMMARY: Bugfixes "Set burnt vegetation map extra to not leave deep pits hidden under ash piles"

Purpose of change

Another obscure side effect of the PR that allows dirt to be blown into craters I overlooked, there's another map extra in the code that relies on bashing terrain that should be leaving shallow pits at most, not deep pits.

Describe the solution

In map_extras.cpp, updated burned_ground_parser so that burnt vegetation checks for deep pits created by wrecking the ground when it creates ash piles, and converts them back to shallow pits. Mainly good because the ash furniture looks odd and in UDP it obscures the pit in a way that might lead to players blundering into one.

Left the section that doesn't place ash piles untouched, since main goal is preventing concealed deep pits, a few open pits being left seems okay to me.

Describe alternatives you've considered

Reverting neat craters because I keep finding obscure side effects.

Testing

1. Compiled and load-tested.
2. Spawned in after temporarily forcing burnt ground to be more commonplace for testing.
3. Confirmed there were no deep pits under ash piles.
4. Checked affected file for astyle.

Additional context

[scarf005]

left: after
right: before

pits are correctly turned into shallow ones.