Improve TerrainLayer loading experience

[Pessimistress]

For #4236

Change List

• Only display a tile if both the mesh and the surface image are loaded
• Fix an update issue in non-tiled scenario when terrainImage is changed
• Restore no texture option in the demo

[coveralls]

Coverage decreased (-0.03%) to 79.856% when pulling 78ed0d1 on x/terrain-optimize into add2cef on master.

[chrisgervang]

Oh cool! Thanks for fixing the bugs.

I could see cases where either the old or the new loading experience is better. For example, it was nice to progressively load on slow connections so that something could be shown quickly. But it is a less ideal experience and could confuse users.

Was there a technical reason to load this way by any chance? I haven’t thought about it.

[Pessimistress]

Was there a technical reason to load this way by any chance?

TileLayer determines the visibility of tiles based on whether their data loaders are resolved.