Optimized with regards to memory and cpu usage. Also added a new argument: It...

Optimized with regards to memory and cpu usage. Also added a new argument: It is now possible to decode an image with a scale factor (where the factor is an integer >=1) for thumbnail creation. Use ([ shrink_factor:factor ]) in the options mapping to do this. Also added a decode_header method.

Rev: lib/modules/_Image_XCF.pmod:1.13
Rev: src/modules/Image/encodings/xcf.c:1.22