SDL: let SDL decide which display resolution to use in fullscreen mode

After creating the fullscreen window (based on the use specific TERMINAL_WIDTH/TERMINAL_HEIGHT),
use the actual window size to calculate TERMINAL_WIDTH/TERMINAL_HEIGHT and ignore the user supplied ones.
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.
Remove non-unicode characters from game.cpp
Submap refactor: Accessors for submap::rad
Merge remote-tracking branches 'dda/pr/7472' and 'dda/pr/7475' into dda-master

* dda/pr/7472:
  Assorted fixes to game::init_ui and dependencies of it.
  SDL: let SDL decide which display resolution to use in fullscreen mode

* dda/pr/7475:
  Submap refactor: Accessors for submap::rad
  Remove non-unicode characters from game.cpp