ofxs: add utility function for OfxRectI and OfxPointD
PushPixel: add check on pixel components
ImageStatistics: add optional input mask

* fix renderScale
* fix input RoI
* add output parameter with the number of pixels used to compute stats
* update minor version. It doesn't break compatibility.

Fix #339
ImageStatistics: add parameters hints
ImageStatistics: add output HSL variance
ImageStatistics: add param to restrict stats to a user region

* add checkbox to enable/disable user region (by default to false)
* upgrade major version: breaks compatibility (by default the user
region is disabled)
Merge pull request #340 from fabiencastan/dev_imageStatistics_addMask

ImageStatistics: Add mask