software renderer: examples/renderer/15-cliprect bouncing

[icculus]

examples/renderer/15-cliprect draws a single texture with a dstrect of NULL to fill the window, with a cliprect sliding around each frame. In the software backend, this is causing the texture to bounce:

Screencast.from.2024-09-26.15-42-14.webm

It should remain stable, like with the OpenGL backend:

Screencast.from.2024-09-26.15-43-29.webm