Refactor and further generalization of particle model

- Collapsed the per-particle data into a single struct, and
  use that to communicate with drawables, too. Let the drawables
  manage allocation of xforms, colors, etc. Helpful for non-atlas
  drawables, and just to keep the effect code simpler.
- Having all of the params in a single struct allows us to move
  the remaining animated behaviors into affectors (color/frame).
- Added SkColorCurve, which works like SkCurve for SkColor4f.
  Use that to create a color affector (rather than simple
  start/end colors in the effect params).
- Also put the stable random in SkParticleState. This is going
  to be necessary if/when we change affectors to operate on all
  particles (rather than one at a time). Still need to move t
  value into the particle struct (or eval it from the lifetime
  params on demand).

Change-Id: Icf39116acbfd5d6e8eb91e9affbd8898d106211d
Reviewed-on: https://skia-review.googlesource.com/c/193473
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed