GTK: Fix wrong direction of zoom when "smooth scrolling" with mouse w… #100

Job	Run time
Cancel Previous Runs	4s
Test Ubuntu	4m 33s
Test Windows	5m 30s
Test macOS	5m 52s
Build Release Windows (OpenMP)	7m 38s
Build Release Windows	7m 47s
Build Release macOS	13m 19s
Upload Release Assets	0s
	44m 43s

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