Wrap BMP header around other image content, especially GIF

[schnaader]

When using PAQ compressors on files processed with the "pdfbmp" switch, images inside PDFs are wrapped with a dummy BMP header so that the compressor can switch to the image model and compress the image both faster and better.

This should also be implemented for GIF image data.

For PNG image data, research is needed. It might also help, but the image data is filtered here (see here), so it might be necessary to unfilter it.

The switch should be renamed to "wrapbmp" after this.