View transform for thumbnails

The image returned to the client in response to a req=tmb request is derived from the composite image by considering the following values: wid=, hei=, attribute::DefaultThumbPix, and attribute::MaxPix.

1. Calculate the view rect - Use wid= or the width value of attribute::DefaultThumbPix for the width of the view rect. Use hei= or the height value of attribute::DefaultThumbPix for the height. The view rect must be fully specified in this step. (Note that the view rect is the same as the layer 0 rect, if no size=is specified for layer 0).
2. Scale the composite - If catalog::ThumbType=Crop, then the composite is scaled to the smallest image possible while still filling the entire view rect; extra image data is cropped off. If catalog::ThumbType= Fit, then the composite is scaled to the largest image possible while still fitting the entire composite into the view rect. If catalog::ThumbType=Texture, the composite is not scaled at all to preserve the resolution specified in catalog::ThumbRes.
3. Fill and crop - The view rect is filled with the bgc= color (or, if not specified, with attribute::ThumbBkgColor). The scaled composite is aligned with the view rect using attribute:: ThumbHorizAlign and attribute:: ThumbVertAlign. The scaled composite is then merged with the filled view rect without further scaling. Any areas of the composite that extend beyond the view rect are cropped off.