Implement correct draw functions for 16-bpp modes

[akx]

While reworking #9772, I noticed 16bpp drawing was piggybacking on draw8... and then it kind of turned out that not all 16bpp modes were drawing things correctly.

This PR:

• moves explicit I;16 draw tests into their own file
• adds (failing) tests covering more draw functions in all of the I;16 modes
• steals the Bresenham detriplication from Speed up rectangle, polygon and line drawing #9772
• implements draw16, the suite of point/hline/line functions for 16-bpp images, with some additional twists and turns of big-endian support (which was also incorrect in another special way).

Along with correctness, this should also yield a bit of performance since draw8 functions don't need to do per-pixel checks for the image mode (and the Bresenham de-triplication commit also adds the option to use hline).

[codspeed-hq]

Merging this PR will improve performance by ×2.4

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 602 untouched benchmarks
⏩ 335 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_draw_lines[1237x811-L]	912.2 µs	380.7 µs	×2.4

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing akx:draw16 (e3266fc) with main (05fcca1)}

Footnotes

1. 335 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[radarhere]

Suggested change

	@pytest.mark.parametrize("mode", I16_MODES)
	def test_horizontal_line_I16(mode: str) -> None:
	img, draw = create_I16_image_draw(mode)
	draw.line((0, 4, 7, 4), fill=I16_INK)
	assert img.getpixel((4, 4)) == I16_INK
	@pytest.mark.parametrize("mode", I16_MODES)
	def test_vertical_line_I16(mode: str) -> None:
	img, draw = create_I16_image_draw(mode)
	draw.line((4, 0, 4, 7), fill=I16_INK)
	assert img.getpixel((4, 4)) == I16_INK
	@pytest.mark.parametrize("mode", I16_MODES)
	def test_diagonal_line_I16(mode: str) -> None:
	img, draw = create_I16_image_draw(mode)
	draw.line((0, 0, 7, 7), fill=I16_INK)
	assert img.getpixel((4, 4)) == I16_INK
	@pytest.mark.parametrize("mode", I16_MODES)
	@pytest.mark.parametrize(
	"xy",
	# horizontal, vertical, diagonal
	((0, 4, 7, 4), (4, 0, 4, 7), (0, 0, 7, 7)),
	)
	def test_horizontal_line_I16(mode: str, xy: tuple[int, int, int, int]) -> None:
	img, draw = create_I16_image_draw(mode)
	draw.line(xy, fill=I16_INK)
	assert img.getpixel((4, 4)) == I16_INK