Correctly interpret RGBA tiff images as premultiplied

The TIFFReadRGBAImageOriented method turns out to return colors with
alpha premultiplied by default. The only reason we pass our own tests
is because we also save the colors incorrectly unpremultiplied.

The patch fixes the format type of the returned images, and explictily
writes the how alpha should be interpreted in the saved files.

Change-Id: Ie1c3881acfe07eae25ca735adf243c1636f656a0
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>