More detailed tracking of errors when d/l images

Easier to troubleshoot in case of wrong redirects,
or plain origin server errors.
New ops, liquid-rescale and gravity.

We use(d) these operations for My Opera and Discover.
Fixes some whitespace problem with crop

Sadly, I don't remember what was the problem anymore.
Refers to our internal commit 39fc429258dcc76554d015a286105289b18487b0.
Added background operation

Useful when converting PNG pictures to JPEG and at the
same time changing the default black color.

Maybe we could sample the picture colors and decide
a suitable background color too.
Fixup background-op.
Background applies correctly now.

It's necessary to apply the background filter early
in the URL chain. If you define a URL like:

    /resize/240x>/background/red/

then the picture will result with a red background
but it will be the original size. I'm not familiar enough
with ImageMagick to understand why at this point, so
just put your background filter at the beginning.

    /background/grey/resize/240x>/

This will work.
Merge branch 'liquid-rescale'
Merge branch 'crop-whitespace-bugfix'
Merge branch 'background-op'

Conflicts:
	src/mod_dims.c
	src/mod_dims.h
	src/mod_dims_ops.c