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This PR simply allows you to override the opacity of a Galley when you
draw it on screen. Last year I opened #3548 and some changes were
requested to the PR, but unfortunately school got really busy and I
wasn't able to apply them. This PR supersedes #3548 and applys the
changes requested in that PR
Closes#3548
StratusFearMe21
Emil Ernerfeldt
Jozef Číž