[TASK] Remove GFX im_imvMaskState and im_negate_mask settings

There are two very weird settings is GFX array that previously
decided on how "masks" are applied to images if combining
multiple images. GFX/im_negate_mask and GFX/im_imvMaskState are
also manipulated during boostrap and depend on each other.

Fiddling with those settings in ImageMagick 6 and GraphicsMagick
with its supported versions, it turns out that they are obsolete
and can be removed completly. The patch does this.

Change-Id: Ic24c877287382f9e718974a0df3577dad2eeaac9
Resolves: #52088
Releases: 6.2
Reviewed-on: https://review.typo3.org/23880
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn