Fixup alpha bitmap generation for Windows.
Fixup alpha bitmap generation for Windows.

This improves upon d6f58fd25eeca84a94528409a05b80aa5172b8b8 by
using the BitmapEx ctor instead of manual initialisation. Several
EMF+ files would otherwise show up horribly mangled on Windows.

Change-Id: I1808cb45818f3f0118653164c83c49d6134b9324
(cherry picked from commit fefab3bb38f55471d9fa9316d1249ad00adefc37)

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core