the TIFFGetField() return for TIFFTAG_EXTRASAMPLES is defined as (count,
types array) [1]. previously the count was being checked rather than the
first element of the array to determine whether the alpha was associated
(pre-multiplied) and the result needed to be unmultiplied.
since:
9273e441 fix TIFF encoder regarding rgbA/RGBA
[1] http://www.libtiff.org/man/TIFFSetField.3t.html
Change-Id: I6e41be9d038fe8afb6d0aa3c8048925dc901113b