[NGRM] - Strange artefacts in 2D maps

[nilscb]

Story to illustrate problem: Maps Layer/"Strange artefacts in 2D maps"
Commenting out the fix there will appear green lines on the map (see figure)

[nilscb]

It is an interpolation issue in the fragment shader.
Even if giving attribute values between 0 and 1, the fragment interpolation can generate values lower than 0 or greater than 1.