Add an FXAA implementation.

FXAA is an antialiasing technique that is applied in a post-processing pass. This implementation:

• Is a single-pass fragment shader.
• Detects edges.
• Searches for edge endpoints.
• Interpolates colors along the edge to give a smooth, antialiased color gradient.
• Detects and corrects sub-pixel antialiasing.

Related to paraview/paraview#16882 (closed)