Rename `ext::Blur` to `ext::CentralLimitBoxBlur` and rework ex 25

[achalpandeyy]

For #61

• ~~I still gotta fix the mirror wrapping case (which I'll fix soon), apart from that it is ready for review~~
• ~~Some changes are exactly the same as #75 i.e. addition of nbl_glsl_workgroupBroadcast for floating point numbers~~

[achalpandeyy]

Merge to master TODOs:

• [x] Merge achalpandeyy/blur with the current master and make ex25 compile and work.
  • [x] Get ex25 running on old API.
  • [x] Merge with master.
  • [x] Separate out ex25 source in the dedicated examples_tests repo. PR
  • [x] Port to new API and make everything work.
• [ ] API modifications:
  • [ ] Rethink default input and output descriptors.
  • [x] Separate out a descriptors.glsl from default_compute_blur.comp.
  • [x] Default barrier api/dispatchHelper.
  • [x] Utility function for creating shaders.
• [ ] Resolve comments after review.

[devshgraphicsprogramming]

be consitent with naming, the macros/defines should have EXT_CENTRAL_LIMIT_BOX_BLUR instead of EXT_BLUR

the nbl_glsl_ext_Blur should be nbl_glsl_ext_CentralLimitBoxBlur as well