Fixed minor compilation issues

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Cleaned WinDOS backend OpenGL(R) support

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Fixed loading of memory pixmap

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Fixed memory canvas alpha blending

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>