operations: improve properties of chekerboard
operations: clean up convolution-matric, still broken...
operations: update categories
operations: kill more cruft in convolution-matrix
operations: less weird property names in color-rotate, and some cleanup
tools: Return unique error code in img_cmp
operations: fix use of "clockwise" in color-rotate
operations: fix copyright in cast-format
operations: improve property descriptions in color-rotate

Also add an enum to replace a totally unclear boolean. In the
processing code, don't bother copying untouched alpha values back to
themselves.
operations: fix handling of hue in color-rotate
operations: coding style cleanup in convolution-matrix
operations: fix convolution-matrix pretty much completely

- use GeglAbyssPolicy instead of selfmade abyss handling; simply pass
  the policy propery to gegl_buffer_get()
- remove all sorts of junk needed for reimplementins abyss
- properly center the matrix on the source pixel
operations: move convolution-matrix from workshop/ to common/
Updated Russian translation
operations: don't abbreviate property names in convolution-matrix
operations: even more cleanup in color-rotate
operations: turn color-rotate into a point filter
operations: move color-rotate from workshop/ to common/
operations: use R'G'B'A float for color-rotate

because that's what the HSV conversion functions expect. Also
clean up stuff a bit more.
operations: add gegl:copy-buffer which allows to take snapshots

of a graph at any point. It works like gegl:write-buffer but is a
filter that outputs its input unchanged.