The existing code attempts to clear the frame by painting in OVER
mode with transparent black - which is a no-op. As a result if you
have many input frames (e.g. you're using a sequence of svg files),
you'll start to see new frames drawn over old frames as memory gets
re-used.

Switch the code to paint using the CLEAR compositing operator,
which fills every channel with 0 values (setting a source colour
is not required).