operations: Remove remark about input being a babl_format_n type

5285d4587 in babl improved color conversions between babl_format_n
and other babl color models, so the op user no longer needs to
worry about this.
gegl: fix description of "threads" property.

It was using tile-height's blurb. Probably a bad copy-paste.
gnomonic-projection: add panorama VR rendering op

Added an operations that performs a gnomonic / equilinear projection of a
equirectangular input image. (Renders virtual camera images from a panorama.).
Add gnomonic-projection.c to POTFILES.in
stereographic-projection: little planet rendering for panoramas

Operation performing little-planet style rendering of equirectangular
panoramas.
fractal-explorer: Avoid a compiler warning by initializing tmpx and tmpy
fractal-explorer: Handle unmatched fractal type

This was originally looked at to fix a compiler warning of nexta
not being initialized.
operations: Fix case typo in enum identifiers
polar-coordinates: Fix warning about use of uninitialized variables

This is ideally fixed by using something like:

  if (inside)
    {
      ud = rx;
      vd = ry;
    }

... but because the gegl_unmap() macro is also used in the
gegl_sampler_compute_scale() macro, the warning persists due to the
variables passed as ud, vd being used there.