Wireframe and Thick-Line Rendering in WebGPU

Demo: https://potree.org/permanent/wireframe_rendering/ (requires at least Chrome 112)

Render an indexed triangle model as mesh, wireframe, wireframe with thick lines, or quads without the need to generate additional buffers for line rendering.

Uses vertex pulling to let the vertex shader decide which vertices to load, which allows us to render indexed triangle meshes as wireframes or even thick-wireframes.

• A normal wireframe is obtained by drawing 3 lines (6 vertices) per triangle. The vertex shader then uses the index buffer to load the triangle vertices in the order in which we need them to draw lines.
  [code]
• A thick wireframe is obtained by rendering each of the 3 lines of a triangle as a quad (comprising 2 triangles). For each triangle of the indexed model, we are drawing a total of 3 lines/quads = 6 triangles = 18 vertices. Each of these 18 vertices belongs to one of three lines, and each vertex shader invocation loads the start and end of the corresponding line. The line is then projected to screen space, and the orthoginal of the screen-space line direction is used to shift the vertices of each quad into the appropriate directions to obtain a thick line.
  [code]
• Quads are obtained by rendering two triangles for each vertex of the mesh. Any six consecutive vertices in the vertex shader load the same vertex of the mesh and place them such that they form a quad. The vertices are first transformed into view space, where they are then offset along the x and y axis.
  [code]