Implicit WebGL extension requirements

[pjcozzi]

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.