Fix floating point rounding issue

The calculated line width is quite important here, because GCodePath
objects may be merged (or not) depending on it, and sometimes the value
would be rounded to a very close value, but not equal. We now use llrint
to make sure we round to the neareset value.

CURA-12038

src/LayerPlan.cpp

@@ -1151,7 +1151,7 @@ void LayerPlan::addWall(
         for (size_t piece = 0; piece < pieces; ++piece)
         {
             const double average_progress = (double(piece) + 0.5) / pieces; // How far along this line to sample the line width in the middle of this piece.
-            const coord_t line_width = p0.w_ + average_progress * delta_line_width;
+            const coord_t line_width = std::lrint(p0.w_ + average_progress * delta_line_width);
             const Point2LL destination = p0.p_ + normal(line_vector, piece_length * (piece + 1));
             if (is_small_feature)
             {