qtcollider: QImage - improve 'formats' implementation

* Optimize array instantiation
* Optimize QByteArray to PyrString conversion
* Perform GC write barrier when filling array with strings
qtcollider: QImage - implement accelerated painting via QPixmap

Automatically switch internal representation between QImage and QPixmap;
the former is used for pixel access and drawing onto image, while the
latter is used when displaying image on screen.