Surfaces in view overlays are not pickable

[pmconne]

Describe the bug A surface graphic containing pickable features does not write to the pick buffers.

To Reproduce Steps to reproduce the behavior:

1. Pull the branch in #7157.
2. Build to core-frontend
3. Run the tests
4. Observe failure:

  Pickable decorations
    ✔ world and overlay point decorations are pickable (1143ms)
    1) world and overlay surface decorations are pickable
    ✔ view background is not pickable (523ms)

  2 passing (5s)
  1 failing

  1) Pickable decorations
       world and overlay surface decorations are pickable:

      AssertionError: expected false to equal true
      + expected - actual

      -false
      +true

      at Decorator.test (home/paul/c/j2/s/itwinjs/core/frontend/src/test/render/webgl/PickableDecorations.test.ts:44:54)
      at home/paul/c/j2/s/itwinjs/core/frontend/src/test/render/webgl/PickableDecorations.test.ts:97:17
      at testBlankViewport (home/paul/c/j2/s/itwinjs/core/frontend/src/test/openBlankViewport.ts:80:7)
      at Context.<anonymous> (home/paul/c/j2/s/itwinjs/core/frontend/src/test/render/webgl/PickableDecorations.test.ts:93:22)

Expected behavior The overlays should be pickable.

Screenshots n/a

Desktop (please complete the applicable information):

• iTwin.js 4.9.0-dev.24

Additional context Discovered in #7091.

Pickable view overlays were initially supported in #4827.