operation-keys: make api to get acces paramspec keys public
gegl-tester: improve default composition

Make it easier to see results of composer operations by adding a transparency
gradient on the left side of standard-input.png, and compositing the result of
the operation on top of a checkerboard.
gegl_coordinate_relative_to_pixel new utility function

Make it easier to write self documenting code for converting between relative
and pixel based coordinates. Given either buffer width or height as argument.
ops: use relative coordinates for center of waves, mirrors and motion blurs

This makes default property values for center properties correspond to center
of buffer. Making default settings and reference compositions improve, the
property meta-data is also changed.
c2g, stress: improve reference compositions
vignette: improve property meta data
hack to disable opencl
map-relative: reference-composition: use perlin noise
tests/compositions: add old gegl splash as composition test
ops/color: add color-primary meta data
rectangle: make defaults render something
text: add sample composition
fill, stroke, path: use GEGL G for reference composition