Realtime Compositor: Rewrite inpaint node

This patch rewrites the Inpaint node in the Realtime Compositor. The old
method suffered from discontinuities and singularities in the inpainting
regions. Furthermore, it ignored semi-transparent areas.

The new method is inspired by a two pass method described by the paper:

  Rosner, Jakub, et al. "Fast GPU-based image warping and inpainting for
  frame interpolation." International Conferences on Computer Graphics,
  Vision and Mathematics. 2010.

In particular, we first fill the inpainting region using jump flooding,
then we apply a variable size blur pass whose size is proportional to
the distance to the inpainting boundary. The smoothed region is then
mixed with the input using its alpha.

The aforementioned method requires variable size blur, which is quite
expensive for this use case, so a new implementation was added that
approximates the method using a separable implementation, which provides
a visually pleasing result assuming a sufficiently smooth radius field,
which is true for our case since the field is an SDF.

Fixes: #114422

@@ -0,0 +36,4 @@
+   * inpainting distance. Additionally, we clamp to the distance to the inpainting distance since
+   * areas outside of the clamp range only indirectly affect the inpainting region due to blurring
+   * and thus needn't use higher blur radii. */
+  float blur_window_size = min(max_distance, distance_to_boundary) / M_SQRT2;