Support using maki as a value for image property

[kkaefer]

Currently we cannot use the variable maki as a value for the image property for poi_label markers. As a workaround, we are creating individual buckets for each maki type, but this is problematic because 1) we cannot also use the scalerank field to then control label/marker density, and 2) it is not practical to make a bucket for every single maki type.

Example showing only parks:

[kkaefer]

It looks like we're going to have to implement a way for updating our buffers after they were uploaded to the GPU. This is necessary because otherwise we'd have to wait until the sprite is loaded before requesting/parsing any tiles, since they are now dependent on the sprite.json file.

[kkaefer]

This just renders all points in the poi vector tile layer, using the maki property for looking up the appropriate texture:

Missing things before this can be merged:

• If the sprite is not yet available (because it is still being downloaded), we need to reparse the layer and assign the correct texture coordinates
• Rendering on retina screens is not working properly
• Make sure we're creating a dedicated dot bucket in case we don't want to assign a texture
• We are currently drawing one fixed size of icons per bucket

Reparsing the tiles seems like a slightly larger effort, so as a quick stopgap, we could change the sprite class so that it is always allocated from the beginning. As we add named icons, we reserve space in the sprite (like the glyph atlas), but use some sort of default icon (e.g. a dot) that we hardcode into the app. When the sprite loads, we're going to decode the image file, and copy the icons over to the correct spot in the sprite image.

We could also move the dot shader to use this system, by reusing the same infrastructure; instead of actual icons, we generate the "dots" on the CPU and use them as textures.