qt: use sub-texture for source instead of filtering when feasible in `PartialEffect`

Providing sourceVisualRect now makes the source visual to use sub-texture instead of filtering out the effect part, which is done when blending is enabled.

When blending is off, since the opaque effect is drawn first, the fragments of the source visual that are obscured by the effect area should already be discarded due to early fragment test, provided that the scene graph uses depth buffer (which is the default). If not, sourceVisualRect can be used to have the same benefit.