[HTML5] Better fullscreen, canvas resizing.

Three canvas resize policies:
- `None`: Godot window settings are ignored.
- `Project`: Godot handles the canvas like a native app (resizing it
  when setting the window size).
- `Adaptive`: Canvas size will always adapt to browser window size.

Use `None` if you want to control the canvas size with custom JavaScript
code.

platform/javascript/display_server_javascript.h

@@ -57,9 +57,6 @@ private:
 	double last_click_ms = 0;
 	int last_click_button_index = -1;
 
-	int last_width = 0;
-	int last_height = 0;
-
 	bool swap_cancel_ok = false;
 
 	// utilities