Make FleX collision events available to Lua

[Derpius]

Context

• FleX emits particle<->shape contacts to a contacts buffer
• This is a fixed length equalling the number of particles, with offsets into a larger contacts buffer (accounting for max 4 contacts per particle)
• We have to perform a linear search of the whole buffer to find all contacts
  • FleX provides a counts buffer which gives the collision counts per particle, so we definitely are O(N) for maximum number of particles, but not maximum number of contacts
• Calling into Lua for every contact would be extremely slow (especially if doing anything intensive on particle contact)
  • Instead, we should aggregate the collisions by shape, and call a collision listener once per shape instead

Scope

todo - I don't know enough of how the binary module is structured to scope this properly @yogwoggf

The algorithm is roughly as follows:

1. Spawn a number of threads
2. On each thread:
   1. Create an unordered map of shape IDs -> collision info
      • If shape IDs are roughly small and consecutive, could use an array of length MAX_SHAPE_ID
   2. For each particle allocated to this thread
      1. For each contact as given by counts[particleIndex]
         1. Get shape ID, force vector, other userdata (e.g. particle type for portal gel type things and material properties)
            • If we don't have a particle userdata buffer yet to store the material and other things, this should be added
         2. Aggregate the data with the thread-local shapeCollisionInfo[contactShapeId]
            • For force this should just be addition
            • For other props you may need a set of unique particle userdatas that were encountered (with e.g. a particle count assigned to each for weighted damage / other effects in lua)
3. After all threads complete, aggregate each thread's local aggregations together using the same algorithm as on the thread

Acceptance Criteria

• Gelly provides addCollisionListener(function(entity, collisionDetails) end) and removeCollisionListener(myFunctionHandle) methods to Lua
• Gelly calls each callback once per entity (shape) that was collided, with details of the collision passed as a table (force, array of particle types, etc.)
• The collisions for a shape are aggregated
• The performance of collison listeners are acceptable
• It's possible to differentiate between particle types, to e.g. allow for portal gel presets to be created without having one or other spawned
  • Should be trivial if we already track materials on a per-particle basis
  • Otherwise we need a particle userdata buffer to be created and updated throughout the simulation

Other

No response