[Bf-codereview] Alpha premul pipeline cleanup (issue 7018048)

[sergey.vfx at gmail.com]

In updated patch there is:

- Alpha mode is set properly to TIFF files. There's one hack in loading
files to prevent color premultiplication in the core of libtiff (which
seems always output premul color, even for bytes)

- Detect alpha mode from file when first time opening it.

- Added compatibility code for sequencer

https://codereview.appspot.com/7018048/