[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/os_javascript.h

@@ -42,7 +42,6 @@
 class OS_JavaScript : public OS_Unix {
 private:
 	VideoMode video_mode;
-	Vector2 windowed_size;
 	bool window_maximized;
 	bool entering_fullscreen;
 	bool just_exited_fullscreen;