Revise handling of GeglColor, formerly GimpRGB.
Marshall to/from Scheme lists of numeric.
Marshall from string CSS names and notations.
Conversion to/from GRAY, GRAYA, RGB, RGBA.
See color.scm test script.
New functions with the same name as these functions are created, except without
the length argument (i.e. it's equivalent to calling these with -1).
The reason for this is that using strings with a length variant which may be
negative to switch to NUL-terminated strings are not bindable. At least in our
case, when testing in Python, the input string ended up as corrupted garbage and
GObject Introspection docs warns about such interfaces:
> In particular, avoid functions taking a const char * with a signed length that
> can be set to a negative value to let the function compute the string length
> in bytes. These functions are hard to bind, and require manual overrides.
(see: https://gi.readthedocs.io/en/latest/writingbindableapis.html#strings)
So instead, I create a simple version which runs on NUL-terminated strings only
and which is bound, whereas unbinding the generic length-version (making it
C-only, or maybe usable in some other bindings which ignore the (skip)
annotation; apparently some do this).
At script create time, create a GimpRGB i.e. pixel from the declared name of default color.
At plugin procedure registration time, convert that pixel to GeglColor,
and use that color to declare the color arg using GeglParamSpecColor(default=color)
Fixes a faulty fix a few commits back where we lost the declared color name.
The proper dataflow is name->color->paramspec-with-default-color.
Extract methods related to color into new file.
So method names document what is being done.
So related code is together.
Ditto for resource.
No functional change.
Preparing for changes to reset/default and changes to representation of pixels
in lists of differing lengths rgb vs rgba.
