QPdfEngine only supports 8 bit/channel images

The only image format supported by QPdfEnginePrivate::addImage() is QImage which is limited to 8 bit/channel RGB. This engine should allow adding images that are higher bit depth (16 bit/pixel for example) since this is supported by the PDF specification and it necessary for those who want to use high quality images in their work flows. For example photographers commonly use 16 bit/channel and some even use 32 bit/channel formats. Printing work flows on OS/X are already PDF centric and Linux is moving in that same direction. SO this needs to be corrected.