Raise TypeError if Viewport is not a Rect and make Viewport

[MiCurry]

This merge makes viewport a property of Camera2D and adds a setter for it. In the setter, and in init, we check that viewport is a Rect and raise a TypeError if it is not.

Without this check, an error will be raised after either calling Camera2D.equalise() or in Camera2D.use(), which may confuse users as to why the error is occurring.

Backtrace when calling Camera2D.equalise() with a non-rect viewport:

File "E:\Projects\SpaceGame\SpaceGame\gamemodes\basegame.py", line 128, in setup_two_player_cameras
    player_one_camera.equalise()
File "E:\Programs\python-arcade\arcade\camera\camera_2d.py", line 336, in equalise
    self._projection_data.rect = XYWH(x, y, self.viewport_width, self.viewport_height)
                                            ^^^^^^^^^^^^^^^^^^^
File "E:\Programs\python-arcade\arcade\camera\camera_2d.py", line 751, in viewport_width
    return int(self._viewport.width)

Backtrace when calling Camera2D.use() with a non-rect viewport:

File "E:\Projects\SpaceGame\SpaceGame\gamemodes\pvp.py", line 139, in on_draw
    self.cameras[player].use()
File "E:\Programs\python-arcade\arcade\camera\camera_2d.py", line 271, in use
    self._window.ctx.viewport = self._viewport.lbwh_int