tile clipping for multiple sources

[ansis]

Problem case with the current approach:

P is a tile, and A, B, C, D are its children.
You have two sources.
The first source has all children: A, B, C, D.
The second source has P and one of A, B, C, D.

You want to draw P from the second source to an area covered by the three other child tiles (from the other source). Its not possible with just an AND and EQUAL to match any three of (11, 10, 01, 00). With a single source you'd never draw a parent to an area with a child tile, but with two sources you might.

kkaefer says:

https://github.com/mapbox/llmr-native/blob/master/src/util/clip_ids.cpp#L72 ==> so this either needs to be always false
in that case, we never assign prefixes that are all-zero
however, that means that a regular zoom level with all tiles loaded takes 3 bits
...or figure out whether all child tiles are from the same source
...
however, with the most recent composite layer push, we're using different stencil buffer objects anyway
so maybe we can just swap the stencil buffer for the current fbo