Assume sRGB when image doesn't have an ICC profile attached

This seems to be no problem with other image viewers and
is apparently also recommended by W3C.

https://bugzilla.gnome.org/show_bug.cgi?id=554498
Allow color correcting images with alpha channel

This is mostly useful for PNG images for now but could also help
with TIFF files in the future. LittleCMS will still ignore the
alpha channel, so this should be harmless.

https://bugzilla.gnome.org/show_bug.cgi?id=725357
Prepare for release 3.11.91
Post-release version bump