Fix bug #1606: the game minimizes when it loses focus in fullscreen

As I suspected, SDL did that on purpose. Fortunately there is a way to
disable it.

Thanks to @gfgtdf for help.

changelog

@@ -64,6 +64,8 @@ Version 1.13.8+dev:
  * Miscellaneous and Bug Fixes:
    * Add --report/-R command line switch for printing the same report from the
      Game Version dialog's clipboard function to stdout.
+   * On GNU/Linux the game no longer minimizes when it loses focus in fullscreen mode
+     (bug #1606)
    * Fixed whiteboard crash on delete action with multiple moves (bug #1842)
    * Fix units beeing unable to step on hexes with too high movecosts (bug #1473) 
  * WFL Engine

players_changelog

@@ -40,6 +40,10 @@ Version 1.13.8+dev:
      their next level-up (issue #1738).
    * Fixed team selection screen often ignoring attempts to scroll (bug #1632).
 
+ * Miscellaneous and bug fixes:
+   * On GNU/Linux the game no longer minimizes when it loses focus in fullscreen mode
+     (bug #1606)
+
 Version 1.13.8:
   * Under the Burning Suns:
      * New sprites for Quenoth Youth (Kaleh and Nym) and Human Commander.

src/sdl/window.cpp

@@ -54,6 +54,10 @@ window::window(const std::string& title,
 	// Set default blend mode to blend.
 	SDL_SetRenderDrawBlendMode(*this, SDL_BLENDMODE_BLEND);
 
+	// In fullscreen mode, do not minimize on focus loss.
+	// Minimizing was reported as bug #1606 with blocker priority.
+	SDL_SetHint(SDL_HINT_VIDEO_MINIMIZE_ON_FOCUS_LOSS, "0");
+
 	pixel_format_ = info.texture_formats[0];
 
 	fill(0,0,0);