Multithreaded SCAssetExportSession

Added player:didSetupRenderer:

Added overlay using UIView

Better support of filters that modify frames in SCAssetExportSession

Improved SCImageView and SCSwipeableFilterView instantiation speed

Added setSelectedFilter: to SCSwipeableFilterView

Removed shared SCContext

Fixed some orientation problem

Added way to change segments directory.

Added progress in SCAssetExportSession