Impress has tools Color that is wrapped in this primitive, then later
drawinglayer::processor2d::VclPixelProcessor2D::processBasePrimitive2D()
converts it back to tools Color. Problem is that the primitive uses basegfx
BColor, so the alpha channel is lost.

Add member and API to survive this roundtrip.

Change-Id: I940e60f6e352022306abac3223636d19dd859355
(cherry picked from commit 9824134a)