Consider updating maximumScreenSpaceError to 1.5 by default

Spun out of #3279

Now that our terrain and imagery system has been upgraded (#7061), would it make sense to lower the `maximumScreenSpaceError` default to something like 1.5? This would increase tile downloads slightly but would also increase visual quality and could better align with what people expect from 2D top-down views.

We should do a head-to-head comparison of various settings and see what the request/performance metrics look like. A slightly tweaked test I used in #7061 should work well for this too: https://github.com/AnalyticalGraphicsInc/cesium/pull/7061#issuecomment-461844082

Start by reviewing the terrain and imagery changes from #7061 and the tweaked test described in its linked comment. Compare several maximumScreenSpaceError settings, including 1.5, and collect tile-request and performance metrics. Done means the comparison supports a default value and the visual-quality trade-off is documented.

Written by the indexing model from the issue text.