Assorted fixes to game::init_ui and dependencies of it.

Added some comment to game::init_ui explaining how the different font dimensions work.
Removed second call to SDL_SetRenderTarget in WinCreate
Renamed translate_terrain_window_size/translate_terrain_window_size_back to (to/from)_map_font_dimension which does match their content better.
Made the from_map_font_dimension function round the result up to include partially visible squares.