Generate the composite "RGB" icon at run-time

One less PNG we have to store, and one more high-DPI-friendly UI element
- yay!
Fix problematic metafile handling in Vista and earlier

This is a damned if I do, damned if I don't situation.  Complications
are extensive, but basically this fix disables automatic alpha detection
of metafiles on Vista and earlier.  Without this fix, GDI+ mistakenly
creates a faulty alpha channel for all metafiles.

Win 7 and later have no such trouble.