Add slice processing to spot_remover filter
Add slice processing to spot_remover filter

Process each channel in a separate thread

Test case with worst case rectanble that fills the image:

no filter:
time ./Shotcut/Shotcut.app/melt noise out=500 -filter volume:0  -consumer null  terminate_on_pause=1 real_time=-1
real	0m1.544s
user	0m1.484s

single thread filter:
time ./Shotcut/Shotcut.app/melt noise out=500 -filter volume:0 -filter spot_remover rect="0% 0% 100% 100%" -consumer null  terminate_on_pause=1 real_time=-1
real	0m5.497s
user	0m5.425s

multi thread filter:
time ./Shotcut/Shotcut.app/melt noise out=500 -filter volume:0 -filter spot_remover rect="0% 0% 100% 100%" -consumer null  terminate_on_pause=1 real_time=-1
real	0m3.745s
user	0m5.585s

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