Port Insta filters to GEGL

[automata]

Bug tracking work on GEGL to implement Instagram inspired filters.

[jonnor]

Basic set of ops reviewed and merged into GEGL. Needs integration and testing on imgflo side.

[jonnor]

This should kinda be in imgflo-server repo, but whatever.

[jonnor]

Tested the GEGL op insta-filter now.
1977 seems directly compatible.
nashville and brannan are significantly different, because the NoFlo graphs change brightness/saturation/contrast in addition to applying a color transfer curve.
hefe and xproii are not available.

[jonnor]

We need to decide whether to a) change the filters to now be without constrast/brightness/saturation changes b) try to implement matching behavior using GEGL existing components c) port the Apply* ops to gegl directly.
Plan is to compare the two sets of filters on a set of images, to see if we prefer a or b/c.

[jonnor]

On small images (150x150 px), with insta_1977, noflo takes 4200 ms and gegl 600 ms. Lots of this is due to component loading startup costs though. With a 9 mpixel, roughly typical of high-end smartphontes. NoFlo takes 9600ms and GEGL 5400 ms.

So there is significant savings available.