cpu_engine: fix an partial rendering flickering issue

when rendering is requested without an update, the buffer is not cleared.
A stale no-dirty flag could cause the existing buffer to be overwritten,
resulting in visual corruption.

This fix resets the task no-dirty flag after each render.
This prevents a task prepared with dirty-region tracking disabled from
bypassing partial rendering on subsequent frames. Now unnecesary
rendering wouldn't be triggered Unless the full rendering
or buffer clear were presented.

issue: #4566