Add 3DTILES_implicit_tiling_multiple_contents extension

[lilleyse]

This extension provides a more efficient way for representing multiple contents in implicit tilesets, especially when tiles have a widely varying number of contents.

It does this through a new tile metadata semantic called TILE_CONTENT_COUNT.

Illustration of multiple contents with content metadata, showing a subtree of depth two with four available tiles. The first tile contains zero contents, the second tile contains two contents, the third tile contains zero contents, and the fourth tile contains three contents. There are five contents total, each having a timestamp and uniqueId associated with it.

This is one of three extension for supporting time-dynamic 3D Tiles: #102 (comment)

• Add 3DTILES_implicit_tiling_custom_template_variables extension #815
• Add 3DTILES_implicit_tiling_multiple_contents extension #816
• Add 3DTILES_time_dynamic extension #817

To do:

• Make github links relative paths
• Link to CesiumJS implementation

[weegeekps]

Can you mix this extension with the existing approach to handling multiple contents?