Add ability to drape the background map onto reality meshes.

[MarcNeely]

This ability already somewhat existed. This PR simply exposes it more so that context reality models which are meshes as well as other reality models which are meshes can have the background map draped onto them as well at will. This is done by adding a RealityMeshDisplaySettings member to the RealityModelDisplaySettings class.

This is a partial fix for issue #6214

[MarcNeely]

@pmconne Paul, the change I'm wondering about is in RealityModelTileTree:addToScene. Currently the display setting for the draping cannot turn off the draping if it is set in the model tile data with the doDrapeBackgroundMap flag. I'm actually not sure how we would tell that they wanted it to override it anyhow since the bgMapDrape flag is off by default in the RealityMeshDisplaySettings unless we change this from a boolean to an enum and provide a third option for overriding what the tiles say.

[pmconne]

@MarcNeely the flag on the tileset indicates a rare, legacy scalable mesh export whose texture imagery comes from a map service instead of being baked in to the tiles. To me, that seems orthogonal to your new RealityMeshDisplaySettings flag that controls whether or not we replace the tileset's texture imagery with map imagery. Kinda like how overriding an element's color to blue would have no visible effect if the element is already intrinsically blue.