Since GDI+ does not return the color depth correctly in many situations, try getting the information with WIC (Windows Imaging Component)

Based on TortoiseSVN revision 24614

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
TortoiseGitIDiff: Fix regressions after changing zoom factor from double to int

Based on TortoiseSVN revision 24615, 24616

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>