Apply "snap 2D transforms to pixel" to viewport

[alvinhochun]

We shall not leave the viewport transform to be rounded by the code for rounding canvas items. Since the viewport transform is inverse to the camera transform, we get incorrect rounding at the halfway point that misaligns the viewport and the canvas item which the camera is following.

Instead, reintroduce viewport rounding, but do it in a way that matches the rounding of canvas items. Also take into account the half-pixel offset of the centre point when viewport dimension is not divisible by two. For CanvasLayers that follows viewport, take into account the scale when rounding. Overall this should work better compared to the rounding in Godot 4.2 (and earlier).

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Fixes #93712

This reimplements the viewport rounding removed from #87297.

I've tested this with the MRP there, with odd and even viewport sizes, and with a bunch of other scenarios with CanvasLayer following viewport and scaled.

[markdibarry]

Tested and can confirm that there's no jitter with odd-sized viewports. This does re-introduce the jitter for Parallax2D, though. Changing _camera_moved() to something like this fixes it, but if someone has a better suggestion I'd like to hear it. (Always annoying to add on more logic per frame if it's not necessary)

void Parallax2D::_camera_moved(const Transform2D &p_transform, const Point2 &p_screen_offset, const Point2 &p_adj_screen_pos) {
	if (!ignore_camera_scroll) {
		if (get_viewport() && get_viewport()->is_snap_2d_transforms_to_pixel_enabled()) {
			Size2 vps = get_viewport_rect().size;
			Vector2 offset;
			offset.x = ((int)vps.width % 2) ? 0.0 : 0.5;
			offset.y = ((int)vps.height % 2) ? 0.0 : 0.5;
			set_screen_offset((p_adj_screen_pos + offset).floor());
		} else {
			set_screen_offset(p_adj_screen_pos);
		}
	}
}

[clayjohn]

@alvinhochun Can you apply the change from Marks comment? After that, this should be good to merge.

[clayjohn]

@markdibarry Can you confirm that this no longer introduces a regression in your testing project?

[markdibarry]

Sorry for the late reply. I didn't notice any regressions with the test project. No jitter for odd sized viewports, parallax or no.
Of course, jitter still remains for drag margins and camera smoothing, but this is a good step forward!

[akien-mga]

Thanks!