GTK: Fix wrong direction of zoom when "smooth scrolling" with mouse w… #18

Job	Run time
Cancel Previous Runs	5s
Test Ubuntu	4m 21s
Test macOS	5m 52s
Test Windows	5m 41s
Build Release Windows (OpenMP)	7m 24s
Build Release macOS	15m 44s
Build Release Windows	7m 27s
Upload Release Assets	0s
	46m 34s

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