Fix gradient regressions

1. Fix regression 8659d189ec04a - rect. gradients no longer do grayscale

In commit 8659d189ec04aca78c8ffff97fcca507ca0a9ec3 I swapped the passing
parameters going to ImplComplexGradient and ImplLinearGradient from
aGradient (the copy) to rGradient (the original which didn't have the
grayscale applied).

2. Fix regression 954d7ad4ea7 - grayscale applied at wrong time

In commit 954d7ad4ea7ec3746b0f0cd3f850a25e82b39c14 grayscale was applied
in DrawGradient (Rectangle &rRect...) after the gradient was written to
the metafile. I was attempting to bring it into line with what
DrawGradient (PolyPolygon &rPolyPoly...) does. However, it remains to be
seen if that's actually doing the right thing - so best to revert this
as in all likelihood I may have caused a regression with grayscaled
gradients

Change-Id: Id94549b3366adb8fcf3cddc59d30029579430a30
Reviewed-on: https://gerrit.libreoffice.org/8908
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>