Under certain conditions, program icons loaded on Windows via GDI+ have
empty mask data, even though the original icon has proper mask data. As
a result, these icons are printed with a black instead of a transparent
background. Still these icons can contain valid alpha data in their
usual 32-bit data.

With this change, alpha data is extracted for icons which are loaded
without proper mask data to ensure that they have proper transparency
information.

Fixes https://github.com/eclipse-platform/eclipse.platform.swt/issues/715
Fixes
https://github.com/eclipse-platform/eclipse.platform.swt/issues/1130