Do not add redundant normalize filters
Do not add redundant normalize filters

When loading a producer from XML, normalize filters are already
added. Do not add them again.

Cause the bug reported here:
https://forum.shotcut.org/t/21-05-18-size-position-rotate-values-not-saved-causes-crashes/27755

j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt