The frame rate of the AVItem, in frames-per-second. This is the reciprocal of the frameDuration. When set, the reciprocal is automatically set as a new frameDuration value.
• In a CompItem, the value is linked to the frameRate of the composition, and is read/write.
• In a FootageItem, the value is linked to the frameRate of the mainSource object, and is read-only. To change
it, set the conformFrameRate of the mainSource object. This sets both the frameRate and frameDuration of the FootageItem.
Floating-point value in the range [1.0..99.0]; read-only for a FootageItem, otherwise read/write.