[Dx9] Incorrect clipping when the destination texture width is not a multiple of the tile size

Artifacts can be seen near the right edge.

<img width="229" alt="image" src="https://github.com/servo/pathfinder/assets/28705694/d5183601-e9b2-4a03-8af0-41f11e07eeea">

For reference: https://github.com/floppyhammer/pathfinder-cpp/issues/21