The branch collided with the Image gizmo branch in four places that are not
real disagreements, only two features reaching for the same line:
- the layer's preview_adjust field and the three new declarations in
TextureDisplacement.hpp sat at the same two insertion points
- the panel's member block sat on top of m_erase_mode, which the other
branch moves to protected
- all three texture getters grew an adjustment clause in the same if, and
one of those ifs is rewritten there too
The first three are moved a few lines away. The cache keys are dropped: the
panel now calls invalidate_adjust_textures() when it moves or reverts the
adjustment, which is less code than three widened keys and leaves the
getters alone. 'cog.svg' and the panel's tests also move to a line of their
own, for the same reason.