Changes to handle MxNx3 images of different numeric classes:
* imread.m: Return the appropriate numeric class. Colour images are of size MxNx3, gray images MxN.
Uses ImageMagick's "identify" to find the image type (b&w/grayscale/colour).
* jpgread.cc: Error handler modified to prevent segfaults. Colour images are returned as MxNx3
matrices, gray images as MxN. Conversion from colour to grey-level is not done anymore (rather use
* pngread.cc: No segfault on loading invalid file types. Handle different depths correctly. Split
into pngread, pngwrite and pngcanvas. Return the appropriate numeric classes and matrix sizes.
* imread.m: Prefer jpgread and pngread to ImageMagick.