The title says it all, move the EWA filter to BLI (currently it's
math_interp.c) and use the function from both BI renderer and the
compositor.
This makes more central place of the algorithm, allowing to have
fixes and optimizaitons synchronized across the two usages.
This also fixes T41440: Displacement in compositing creates holes
Reviewers: campbellbarton, lukastoenne
Reviewed By: lukastoenne
Maniphest Tasks: T41440
Differential Revision: https://developer.blender.org/D748
afa3469288