Fix regression in commit 3f84fe1.

Letting movit.crop pass along mlt_image_glsl causes too early conversion
to glsl (in the convert step after the producer's get_image()). The
deinterlace and fieldorder filters that sit between the producer and
movit.crop can place a new image on the frame and change the output
format _after_ calling mlt_frame_get_image(), which causes yet another
conversion to glsl.