MVT: Add support for annotations

[gkjohnson]

• Create a dedicate MVT Annotations plugin that takes an overlay instance (usable via ImageOverlayPlugin)
• Track annotations (road names, locations) with a "locking" mechanism that prioritizes data from the higher LoD annotations.
• Each annotation will have a unique ID constructed for the content + quantized location
• The plugin will use tile visibility to determine which tile content is visible and track the potential annotations across the scene.
• Users can provide a "sort" callback that can be used to sort visual priority of annotations (default set to vertical positioning)
• Road names candidate locations are determined ahead of time.
• Screen space occupation grid (granularity TBD, possibly determined by annotation size) is used to detect and prevent "clashing" of annotations.
• Positioning / settling of annotations is done with raycasting spread over multiple frames.
• Text is done with a text glyph atlas and reference counting to determine whether the space is free. To start we can use a canvas set to the target resolution (no SDF) and instancing via custom shader.