Fix pixels-mocks crate

[parasyte]

We have cargo doctests enabled in CI, but many of them do not run because they are tagged with no_run. The reason for this is the pixels-mocks crate doesn't generate valid code (it just compiles).

This has resulted in at least one bad example that would have hit an assertion failure:

pixels/src/lib.rs

Lines 352 to 367 in de21862

	/// ```no_run
	/// # use pixels::Pixels;
	/// # let surface = wgpu::Surface::create(&pixels_mocks::RWH);
	/// # let surface_texture = pixels::SurfaceTexture::new(1024, 768, surface);
	/// const WIDTH: u32 = 320;
	/// const HEIGHT: u32 = 240;
	///
	/// let mut pixels = Pixels::new(WIDTH, HEIGHT, surface_texture)?;
	///
	/// let pixel_pos = pixels.clamp_pixel_pos((-19, 20));
	/// assert_eq!(pixel_pos, (0, 20));
	///
	/// let pixel_pos = pixels.clamp_pixel_pos((11, 3000));
	/// assert_eq!(pixel_pos, (11, 240));
	/// # Ok::<(), pixels::Error>(())
	/// ```

The last assertion should compare against (11, 239)

Fixing the pixels-mocks crate enough that the examples can run would have prevented this error.