Avoid potential DLL hijacking risks by not calling LoadLibrary() to
load gdiplus.dll. As gdiplus.dll is a WinSxS (side-by-side) DLL we
couldn't use the full path anyway as we don't know it.

So just link to the GDI+ functions directly. Gdiplus.dll should be
present on all Windows versions we support anyway. Some complexity
added as MinGW doesn't come with an import library for gdiplus.dll, so
we have to create one ourselves. But we call relatively few functions
from gdiplus.dll so that is not a big deal.