UI: Make marquee selection of line segments precise

Before marquee selection worked by checking whether the Axis Aligned
Bounding Box of an entity and the selection marquee overlap. This selects
(slanted) line segments even though the marquee did not "touch" them.

To fix this for line segments actually check that the selection marque
intersects the line segment.

src/draw.cpp

@@ -220,6 +220,21 @@ void GraphicsWindow::SelectByMarquee() {
         bool entityHasBBox;
         BBox entityBBox = e.GetOrGenerateScreenBBox(&entityHasBBox);
         if(entityHasBBox && entityBBox.Overlaps(marqueeBBox)) {
+            if(e.type == Entity::Type::LINE_SEGMENT) {
+                Vector p0 = SS.GW.ProjectPoint3(e.EndpointStart());
+                Vector p1 = SS.GW.ProjectPoint3(e.EndpointFinish());
+                if((!marqueeBBox.Contains({p0.x, p0.y}, 0)) &&
+                   (!marqueeBBox.Contains({p1.x, p1.y}, 0))) {
+                    // The selection marquee does not contain either of the line segment end points.
+                    // This means that either the segment is entirely outside the marquee or that
+                    // it intersects it. Check if it does...
+                    if(!Vector::BoundingBoxIntersectsLine(marqueeBBox.maxp, marqueeBBox.minp, p0,
+                                                          p1, true)) {
+                        // ... it does not so it is outside.
+                        continue;
+                    }
+                }
+            }
             MakeSelected(e.h);
         }
     }