fix scrolling in message window
Replace 'g->m.' with (implicit) 'this->'

The lightmap code would only operate on the g->m map, but it should actually operator on the 'this' object.
Similar in map.cpp
And in field.cpp
map::add_field should only call step_in_field when the this object is the main map.

This prevents fields that are spawned in a tinymap during mapgen to trigger that function.
Merge remote-tracking branch 'BevapDin/fix-message-window-scrolling' into FrankenMod

Conflicts:
	src/messages.cpp
Merge remote-tracking branch 'BevapDin/remove-indirection' into FrankenMod