render_con: Get rid of render_fullscreen argument

Only true for the fullscreen container and doesn't affect any of its
children. Thus, we can get the same result by checking
->fullscreen_mode.

include/render.h

@@ -40,7 +40,7 @@ typedef struct render_params {
  * updated in X11.
  *
  */
-void render_con(Con *con, bool render_fullscreen);
+void render_con(Con *con);
 
 /**
  * Returns the height for the decorations