Vello renders image with a half-pixel offset

Drag an image into the viewport and view it at 100% scale. Toggle the Vello renderer on and off. Notice that it shifts down and left when Vello is active, and that it becomes blurry in the Vello version because anti-aliasing is applied to do the shifting. Also at slightly different from screenspace-aligned integer scales, we have the bottom and right edges wrap and appear as a 1px strip along the top and left edges.

https://github.com/user-attachments/assets/8e4a4916-61ca-49c5-aed5-062d72cbc271

My hypothesis is that between SVG and Vello, one assumes pixel coordinates are at the center of a pixel vs. the top left of the pixel.

Reproduce the issue by dragging an image into the viewport, viewing it at 100% scale, and toggling the Vello renderer. Compare the SVG and Vello rendering paths, focusing on pixel-coordinate conventions and behavior near integer scales. Done means the image remains aligned and sharp, without wrapped 1px edge strips.

Written by the indexing model from the issue text.