Default to fullscreen on, tweak default window res

It makes more sense as a game to start with fullscreen enabled. As for the default res,
it's unlikely to be used (only crops up if the saved resolution value is invalid), but
it makes sense to use a more modern widescreen resolution.

src/preferences/general.cpp

@@ -67,8 +67,8 @@ namespace preferences {
 const int min_window_width  = 800;
 const int min_window_height = 600;
 
-const int def_window_width  = 1024;
-const int def_window_height = 768;
+const int def_window_width  = 1280;
+const int def_window_height = 720;
 
 const int min_font_scaling  = 80;
 const int max_font_scaling  = 150;
@@ -395,7 +395,7 @@ bool maximized()
 
 bool fullscreen()
 {
-	return get("fullscreen", false);
+	return get("fullscreen", true);
 }
 
 void _set_resolution(const point& res)