feat(gray-scott): add reaction-diffusion demo

Add a Gray-Scott page with a shader ping-pong on float textures,
five presets and pointer painting.
Tick the Gray-Scott item in ROADMAP.md.

src/app/constants/UrlConstants.ts

@@ -25,6 +25,8 @@
   static readonly PARTICLE_LIFE_WIKI = 'https://particle-life.com'
   static readonly PHYSARUM_WIKI = 'https://en.wikipedia.org/wiki/Physarum_polycephalum'
   static readonly PHYSARUM_PAPER = 'https://doi.org/10.1162/artl.2010.16.2.16202'
+  static readonly GRAY_SCOTT_WIKI = 'https://en.wikipedia.org/wiki/Reaction%E2%80%93diffusion_system'
+  static readonly GRAY_SCOTT_SIMS = 'https://www.karlsims.com/rd.html'
   static readonly GPU_COMPUTING_WIKI = 'https://en.wikipedia.org/wiki/General-purpose_computing_on_graphics_processing_units'
   static readonly DATA_STRUCTURE_WIKI = 'https://de.wikipedia.org/wiki/Datenstruktur'
   static readonly FOUR_COLOR_THEOREM = 'https://de.wikipedia.org/wiki/Vier-Farben-Satz'