rendering: Do not offset vertices for drawing polygon edges

Render surface with edges uses a geometry shader to draw a triangle strip at the edges of polygons.

Apart from calculating the edge equation, it was offsetting the vertex positions to accommodate wide lines.

However, this would lead to overlapping inner edges that become prominent with opacity < 1. This change removes this offset.

Before	After

Note that with this change, mesh boundary edges will be half as wide as the inner edges.