Fix crash when source image stride is not multiple of 16.

Reported in Kdenlive bug 3143:
https://bugs.kdenlive.org/view.php?id=3143
Cleanup indenting in rgbparade.c.
Fix memory leaks in rgbparade.
Cleanup indenting in vectorscope.c.
Fix memory leak in vectorscope.
Fix crash when filter stride is not a multiple of 16 bytes.
Fix possible divide by zero.
Fix memory leaks in cairoblend and cairoaffineblend.