Support many imagery layers with transformations and terrain

* Add the ability to apply a custom transform/projection to an imagery layer, and entities/primitives, including polygons, polylines, billboards, labels, and points. Support all standard (EPSG) coordinate systems.
* Add capability to load many (tens of thousands) of imagery layers in the same view, each with a transformation matrix.
* Drape transformed imagery over terrain.

No files, tests, or entry points are named. Start by surveying the imagery-layer, entity/primitive transformation, coordinate-system, and terrain subsystems to determine the affected APIs and rendering paths. Done would require the requested transformations, standard EPSG systems, large imagery-layer counts, and terrain draping, with coverage for each stated capability.

Written by the indexing model from the issue text.