* moved binning clipping code to the transform stage (-> all triangle clipping and culling is now directly done in the transform stage)
 * instead of flagging a triangle as culled or non-culled, simply set its depth to INFINITY and test for this (frees up one uint in the internal transform data struct)
 * compute min/max x/y bounds in the transform stage for now (at a later point it would be a good idea to output the triangles 2D screen coordinates instead and to compute the bins more precisely, where applicable)