Fix tileset style recompilation edge cases

[lilleyse]

Fixed two cases where styles would be reapplied needlessly every frame

• When the tileset has a style and tileset.preloadWhenHidden is true and tileset.show is false
• When tileset.style is set to an identical style. This wasn't the original bug I was looking into but it was a slowdown waiting to be noticed.

There was some code in Cesium3DTileStyleEngine that only reset styleDirty during the render pass. This is bad because the render pass doesn't get called at all if tileset.preloadWhenHidden is true and tileset.show is false and so the style would forever be dirty, which causes shader recompilations to happen for each tile every frame.

The proper way to do pass-invariant updates is with postPassesUpdate. It ended up being a pretty simple fix.

For testing:

• preloadWhenHidden Sandcastle - observe that content.applyStyle(this._style); is no longer called every frame.
• Identical style Sandcastle - same

Both added unit tests will fail in master and pass in this branch.

I haven't yet tested this in the application code where this was first noticed, but maybe @IanLilleyT can take over that. Then we could also do a performance comparison.

CC @mramato @IanLilleyT

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[IanLilleyT]

I confirmed that the changes and tests work. It wasn't that the shader was being recompiled, it was from building the final shader string and querying the shader cache. Apparently this is a pretty heavy operation, as seen below.

Before:

After:

Thanks @lilleyse !