Implicit WebGL extension requirements

The following tests check for implicit WebGL extension requirements:
* `Scene/Primitive renders with distance display condition per instance attribute` (floatingPointTextures)
* `Scene/GroundPrimitive renders with distance display condition per instance attribute` (floatingPointTextures)
* `Scene/BillboardCollection renders more than 16k billboards` (instancedArray)

The first two should be have a fallback in the display condition implementation (would have expected this to be in #4700) and the last one should also have a fallback in the implementation that does not require instancing and uses multiple draw calls instead of uint32 indices.

Workarounds were added to the tests in #4986.

Inspect the three named Scene tests and the workarounds added in #4986, then review the display-condition and billboard rendering implementations involved in those tests. Done means the tests no longer require floatingPointTextures or instancedArray extensions, while preserving the requested fallback behavior.

Written by the indexing model from the issue text.