gegl, test-gegl: Split out the code to convert a GeglBuffer to a format
This code is necessary whenever the validity of a floating point GeglBuffer has to be asserted because floating point colour component values are inherently not reproducible. Minor errors can creep in due to differences in computer architecture, varying Babl conversions or other changes in code. These errors don't indicate a bug and will cause spurious test failures. Therefore, it's better to convert such GeglBuffers into integer values before checksumming. eg., this is relevant when a buffer is zoomed by a GeglSampler because the result is floating point premultiplied alpha.
Subsequent commits will add a new codec API for GeglBuffer similar to those for GdkPixbuf, which is going to offer on-the-fly zooming while decoding a bitstream. Therefore, this will be useful for testing those code paths.