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 comment said that it used the larger of dFdx and dFdy, but that wasn't in fact how the code was implemented).

The new code adds dFdx and dFdy together 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).

Closes: #18973 (closed)
Backport: release
Backport: paraview/release