LibSoftGPU: Optimize clipping code

Three optimizations are applied:

1. If the list of vertices to clip is empty, return immediately after
   clearing the output list.

2. Remember the previous vertex instead of recalculating whether it is
   within the clip plane.

3. Instead of copying and swapping lists around, operate on the input
   and output lists directly. This prevents a lot of `malloc`/`free`
   traffic as a result of vector assignments.

This takes the clipping code CPU load from 3.9% down to 1.8% for
Quake 3 on my machine.

Userland/Libraries/LibSoftGPU/Clipper.h

@@ -29,8 +29,7 @@ public:
     void clip_triangle_against_frustum(Vector<GPU::Vertex>& input_vecs);
 
 private:
-    Vector<GPU::Vertex> list_a;
-    Vector<GPU::Vertex> list_b;
+    Vector<GPU::Vertex> m_vertex_buffer;
 };
 
 }