So nTransparency is actually also in percent, i.e. max is 100,
instead of the full byte range, like alpha colors in Qt and VCL.

Fixes "SAL_USE_VCLPLUGIN=qt5 ./bin/run vcldemo --show poly"
transparency.

Change-Id: I723cbb2d6b5af4a37b18d3e6a5e0c5b81421221a
Reviewed-on: https://gerrit.libreoffice.org/58789
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>