Make wireframe normal computation more robust

This allows wireframes to render correctly on linux with amdgpu
drivers.  The previous code always used dFdx as the line vector,
unless dFdx.x was equal to zero.  The new code adds dFdx and
dFdy to get the line vector, after using dot product to get them
both pointing in the same direction (since they might be pointing
in opposite directions along the line).