Cell background color diffusion

[o-sdn-o]

The outlines of object boundaries (cell::id) must be set by pro::shape when rendering each window.

A special case of #164 when the background (or image fragment) consists of only one pixel (RGBA color)

struct cell
{
  // Neighbor cells
  //    c0 c1
  //    c2
  //
  // BG interpolation type (two 1-bit values):
  // 0 -- None
  // 1 -- Cubic
  //
  // 0 1
  // │ └────── interpolation type between `c0` and `c2`
  // └──────── interpolation type between `c0` and `c1`

  byte interpolation_vector : 2;
  ...
  rgba fg;
  rgba bg;
  ... other cell data;
};

Region without background interpolation (nearest neighbor)

Region with bicubic background interpolation (I drew in a graphic editor)