Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
We now allow booleanization of all types. This means that empty versions
of all types now evaluate to false. So a Vector2(0,0), Dictionary(),
etc.
This allows you to write GDScript like:
if not Dictionary():
print("Empty dict")
Booleanization can now also no longer fail. There is no more valid flag,
this changes Variant and GDNative API.
Previously godot_variant_new_object constructed Variant without
accounting for the fact that the Object can be a Reference, so refcount
was not increased and References were destructed prematurely.
Also, Reference::init_ref did not propagate refcount increment to the
script instance, which led to desync of refcount info on the script
side and Godot side.
