Firefox doesn't support a `Framebuffer` with `colorTextures` of different `pixelDatatype`

See https://github.com/AnalyticalGraphicsInc/cesium/pull/6069#discussion_r158379454.

This is a post-processing step that would ideally use a `float` texture as a depth gbuffer and a `UNSIGNED_BYTE` texture as a color gbuffer, which seems to work fine in Chrome but not in Firefox.

In context:
https://github.com/likangning93/cesium/blob/aad2105966e756bc86ffff052531cd84af3415e1/Source/Scene/PointCloudEyeDomeLighting.js#L140

Start with the linked discussion in PR 6069 and inspect PointCloudEyeDomeLighting.js around line 140 in the referenced revision. Reproduce the post-processing framebuffer behavior in Firefox and Chrome, then verify that using float and UNSIGNED_BYTE color textures together works consistently without regressing the described depth and color g-buffers.

Written by the indexing model from the issue text.