Describe the bug
GPS Graphic uses a wrong scaling for the background image. The scale is calculated by the distance of the GPS points. But the maps build from tiles usually don't use the gps coordinate system ESPG4326 / WG84, but the transformation EPSG3857. For the x-axis this doesn't matter, but y-axis is scaled differently.
In the current implementation this leads to the gps track being correctly positioned in x-axis, but incorrect in y-axis if the background image is a map from for example Open Street Map.

To Reproduce
Take a map from Open Street Map and try to match it to the gps track. It will be off on the upper and lower end if it matches in width.

Expected behavior
Perfect match of gps track and the map

Desktop (please complete the following information):

• MLT/melt version: 7.34.1 and still in git

Additional context
I implemented a transformation for the latitude, which compensates the different projection of EPSG3857. At least for me the map is now a perfect match. Projection should be done only once when loading / interpolating the file, so it shouldn't affect rendering speed. Hope I hit all relevant lines and didn't mess up anything.

gpsgraphic.patch