Failing: Scene/GlobeSurfaceTileProvider can render more imagery layers than the available texture units

`Scene/GlobeSurfaceTileProvider can render more imagery layers than the available texture units` is failing in Firefox: http://localhost:8080/Specs/SpecRunner.html?spec=Scene%2FGlobeSurfaceTileProvider%20can%20render%20more%20imagery%20layers%20than%20the%20available%20texture%20units

> Failed: promise rejected: RuntimeError: Program failed to link. Link log: Programs with more than 16 samplers are disallowed on Mesa drivers to avoid crashing.

Start by running the named Scene/GlobeSurfaceTileProvider spec in SpecRunner on Firefox and reproduce the Mesa driver link error. Trace how the test renders imagery layers and determine the change needed for the spec to pass without triggering the more-than-16-samplers limit.

Written by the indexing model from the issue text.