MCGR: ColorSteps handling moved to tooling

Due to the fact that handling of a vector GradientSteps
will get used more often with ongoing changes I moved
some handling to tooling, see sortAndCorrectColorSteps.
That method sorts and corrects a vector of ColorSteps
so that only valid entreis remain in a sorted order,
for details please refer to the docu at function
definition. I took the occasion to rearrange that to
work on the single provided vector which is better for
speed & ressources.

Also changed the ColorStep constructor to not
automatically correct constructed entries: While that
is formally correct, it moves an invalid entry to 0.0
or 1.0, thus creating additional wrong Start/EndColor
entries. Those may then 'overlay' the correct entry
when corrections are applied to the vector of entries
which leads to getting the wanted Start/EndColor to be
factically deleted, what is an error.

Also rearranged FillGradientAttribute to now work
initially with ColorSteps, no longer requires the
Start/EndColor, also adapted all usages. This securely
allows start/end and in-between single-color sections
in gradients.

Also needed to re-formulate GradientRect &
GradientElliptical ::appendTransformationsAndColors
method for correct 2D mapping(s) - always incrementing
from the start to end conflicts with the adaption of
the start value for the inner loop mainly because
increment is at start of inner loop (pre-increment).

Corrected errors from clang plugin, but also some
wrong initialiations for basegfx::ColorSteps.

Change-Id: I292592ed9abcfa591f68a680479f4fcdda46cbeb
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/148196
Tested-by: Jenkins
Reviewed-by: Armin Le Grand <Armin.Le.Grand@me.com>

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