[escher] Purge PaperRenderer and associated code

Delete PaperRenderer and every subsystem that was built exclusively
to support it.  Flatland and its RectangleCompositor are the active
production primitives, and they do not depend on any of these
features.

Note: shader code associated with PaperRenderer was *not deleted*.
It is load-bearing in tests.

Highlights of the purge include:
- `escher/paper/`: Deleted the PaperRenderer implementation.
- `escher/renderer/`: Removed the complex 3D RenderQueue
   system (sorting, batching, instancing).
- `escher/scene/`: Deleted the legacy 3D scene graph types
  (Camera, Model, Object, ViewingVolume, Lights).
- `examples/escher/waterfall` & `rainfall`: Deleted the main
  PaperRenderer consumer demos.
- `escher/hmd/`: Removed VR pose buffer latching logic.
- `escher/geometry`, `math`, `shape`: Removed obsolete
  bounding box math, matrix lerps, and mesh builders
  (e.g., RoundedRect).
- `debug_rects` and `escher/shaders/compute`: Removed obsolete
  debug utilities and compute shaders.
- added/updated README.md files to clarify the historical context
  of the surviving offline shader compilation paths.

Bug: fxb/494718197
Test: fx build && fx test escher_tests escher_examples_test
Change-Id: I7c277890896fc61369d7d209182d0c58d0937794
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1547405
Reviewed-by: Yilong Li <liyl@google.com>
Fuchsia-Auto-Submit: Josh Gargus <jjosh@google.com>
Commit-Queue: Josh Gargus <jjosh@google.com>