The diff mode (option in left toolbar) was never a differential display mode.
It is actually an option to force the alpha channel (opacity) of layer colors
to a given value.
So change the name and tool tips to force opacity mode, and make the opacity
parameter adjustable in preferences.
Gerbview: cleanup: remove a old and dead code.
Adds the ability to flip (mirror) the display, just like "Flip
Board View" does in Pcbnew.
Also added this to vertical toolbar since it's handy to not
have to go into the menu system to flip the display back and
forth.
Fixes issue https://gitlab.com/kicad/code/kicad/-/issues/2501
jean-pierre charras
Jeff Young
PJM