Uses cpp macro magic instead. Can now use GEGL_OP_C_SOURCE
to include source in operation, or GEGL_OP_C_FILE to not
include it.

Allows GEGL applications to show the code, so users can
study them or create new operations based on them easily.

The original scale-size requires to pass both height and width.
scale-size-keepaspect will calculate one from the other, preserving
the original aspect ratio if only one is specified.
If no dimensions are specified, keeps original dimensions.

Peeks inside at the content-type to determine loader to use.
Tested for png and jpg files.

Relies on the extension just like file paths. Not ideal, but
tricky to do much better without fetching URI and checking
content-type (could be done for HTTP for instance).

Special string property which is a URI/URL (incl dataURL).
Annotated differently so that user interfaces can show a
dialog particular for picking URIs.

Remove linear buffers and use GeglSampler for input buffer and GeglIterator for output buffer.