raster tile rendering

[incanus]

We need this as it exists on WebGL. If this were together, I could more easily start framing out the iOS-specific API most like our current SDK, possibly merging the SDK, MBXMapKit, and llmr for eventual subsystem replacement with vector rendering.

[incanus]

Work happening in https://github.com/mapbox/llmr-native/tree/raster-work