Fix bitmap glyph skew origin

[devunt]

Bitmap glyphs use Y-down coordinate system (top at y=0), while outline/COLR fonts use Y-up (baseline at y=0). When glyph_transform (skew for synthetic italic) is applied to bitmap glyphs, the skew origin is at the top-left corner instead of the baseline, causing the italic effect to lean in the wrong direction.

Fix

• Negate the shear coefficients to account for Y-down coordinates
• Apply the skew around the bottom of the image (y=height) instead of the top (y=0)

Screenshots

[DJMcNab]

I think it would probably be easier to review if you merged this and #1306 into a single PR, as they're touching very similar code.

Would you be willing to add some tests here? I believe that the existing test in

vello/vello_tests/tests/emoji.rs

Lines 160 to 173 in a04070a

	#[test]
	#[cfg_attr(skip_gpu_tests, ignore)]
	fn little_bitmap() {
	let font_size = 10.;
	let scene = encode_noto_bitmap(TEXT, font_size);
	let params = TestParams::new(
	"little_bitmap",
	(font_size * 10.) as _,
	(font_size * 1.25).ceil() as _,
	);
	snapshot_test_sync(scene, &params)
	.unwrap()
	.assert_mean_less_than(0.001);
	}

has most of the infra you'd need, and just needs to be copied to add a more interesting transform. You'll need to have git lfs installed.

My gut reaction here is that a much simpler solution is possible, possibly to both issues. But for awareness, I haven't looked too closely. Essentially, the issue in both cases is that the glyph_transform is applied before or after another transform in the pipeline, so the transform doesn't apply properly. I just want to double check that you have confirmed that there aren't any solutions of that form for this issue.

This is related to #1273, but there's nothing actionable to be taken from that PR here.