Imagery tiles requested when hidden behind other imagery

[lilleyse]

Imagery tiles are requested regardless of their visibility. If an imagery layer is below another opaque imagery layer its tiles will still get requested and rendered. Is it possible to detect this and not request those tiles?

Example:

var viewer = new Cesium.Viewer('cesiumContainer');
viewer.imageryLayers.addImageryProvider(Cesium.createTileMapServiceImageryProvider({
    url : Cesium.buildModuleUrl('firstlayer')
}));
viewer.imageryLayers.addImageryProvider(Cesium.createTileMapServiceImageryProvider({
    url : Cesium.buildModuleUrl('secondlayer')
}));

[pjcozzi]

Sometimes this is a "feature" to "prefetch" imagery layers, but I agree this would be a welcome optional optimizations that likely defaults to true. This dates back to #526.

[likangning93]

Robustly deciding that an imagery tile is occluded is hard because the tile may have alpha and/or it may be in a weird custom projection (soon... hopefully).

But an approximation that helps determine priority is a lot more doable.

Some thoughts:

• use an rbush of imagery layer rectangles, we can compute which rectangles collide with an imagery tile
• from this we can compute how much of the tile's rectangle is occluded, this probably involves a bunch of recursive Rectangle intersection ops or something
• or just prioritize based on Z-index when there are collisions at all?