[Bug] g.gui.mapswipe flickers when dragging slider

[mankoff]

version=7.8.4 date=2020 revision=exported build_date=2020-12-19 build_platform=x86_64-pc-linux-gnu build_off_t_size=8 libgis_revision=2020-12-19T12:38:20+00:00 libgis_date=2020-12-19T12:38:20+00:00 proj=7.2.0 gdal=3.2.0 geos=3.9.0 sqlite=3.31.1

[mankoff]

NOTE Flicker does not occur if you drag by the circle button

https://user-images.githubusercontent.com/145117/103895409-3d1bf380-50a5-11eb-9cee-14f1a3bb0f9d.mp4

[tmszi]

Interestingly, when you use the slider circle button, the flickering doesn't occur.

[veroandreo]

Just tested and it happens in 7.8.5 and 7.9, as well