1) ofxhClip: make getPixelDepth setPixelDepth getComponents

setComponents virtual so they can have a MT-safe implementation.

2) ofxhClip: add a setAspectRatio method to be consistent with other
clip preferences such as pixel components and pixel depth. However, it
doesn't have a base implementation and is left pure virtual as the
getter method is.

2) ofxhImageEffect: make continuousSamples setContinousSamples
isFrameVarying setFrameVarying setOutputFrameRate getOutputFrameRate
setOutputPremultiplication getOutputPreMultiplication setOutputFielding
getOutputFielding virtual functions so that the host can provide a
MT-safe implementation (all clip prefs variable basically are subject
to asynchronous use).

3) In getClipPreferences() prefer using the set* methods to set the
preferences which can therefore guarantee thread-safety if the host
overrides them.

4) getClipPreferences() : use the new setAspectRatio method of the clip