* Adds optional vec4 USERDATA1 .. USERDATA6 to particles, allowing to store custom data.
* This data is allocated on demand, so shaders that do not use it do not cost more.
Sets `AlignOperands` to `DontAlign`.
`clang-format` developers seem to mostly care about space-based indentation and
every other version of clang-format breaks the bad mismatch of tabs and spaces
that it seems to use for operand alignment. So it's better without, so that it
respects our two-tabs `ContinuationIndentWidth`.
-Enable the trails and set the length in seconds
-Provide a mesh with a skeleton and a skin
-Or, alternatively use one of the built-in TubeTrailMesh/RibbonTrailMesh
-Works deterministically
-Fixed particle collisions (were broken)
-Not working in 2D yet (that will happen next)
* Particle shaders now have start() and process()
* Particle collision happens between them.
* The RESTART property is kept, so porting an old shader is still possible.
This fixes the problem of particle collisions not functioning on the first particle frame.
-Used a more consistent set of keywords for the shader
-Remove all harcoded entry points
-Re-wrote the GLSL shader parser, new system is more flexible. Allows any entry point organization.
-Entry point for sky shaders is now sky().
-Entry point for particle shaders is now process().
