fix: mountable terrain usable for firing heavy weapons agian

[chaosvolt]

Purpose of change

This sets it so mounds of dirt, open windows, and other terrain can support bipods and heavy weapons again.

Fixes #4126

Describe the solution

In ranged.cpp, updated the definition of is_mountable to check for passable tiles OR climb difficulty when screening out closed-off vehicle tiles from the check.

It seems that stuff that doesn't bother with any climb flags due to being a plain passable terrain, like mounds of dirt for example, are most likely keeping the inherited default climbing difficulty of INT_MAX, hence why the check borks because that value is definitely gonna be greater than 5.

Describe alternatives you've considered

Pestering @scarf005 to fix it instead.

Testing

1. Compiled and load-tested.
2. Confirmed I can once more fire an M2 Browning while on or adjacent to open windows and mounds of dirt.
3. Tested a sandbag barricade to confirm that I can still use mountable furniture too.
4. Tested editing a sandbag wall to count as mountable, confirmed it's not valid to mount heavy weapons on due to being impassable and not climbable.
5. Spawned in RV, still barred from using closed doors and boards to mount heavy weapons, but able to when the door is opened.
6. Checked affected file for astyle.

Additional context

Discovered while working on #4129

Checklist

[scarf005]