Fix for alpha channel rendering for images on OS X Mavericks; thanks to Marc Culler for the extensive patch.
Fix for display of images when scrolling a text widget on OS X; thanks to Marc Culler for patch
Add copyright notice to Marc Culler for extensive patch to alpha rendering on Mac/Cocoa; remove private API calls to comply with platform requirements.
Fine-tune scrolling, especially of text widgets with embedded windows, after removal of private API calls; performance is now better and within acceptable ranges.