vector layer: Reduce memory needs of geometry_buffer #223
Description
idea:
currently we use quite a lot of video ram (ca 2 gigabytes). with the fallback, we don't actually need the full geometric data for vector rendering, only the data until the corresponding max drawing id). we can separate the data into live drawing data (used to actually draw the vector geometry) and the tail for the fallback texture. this tail data can be thrown away once the fallback is drawn.
The main problem with this idea is, that currently we allow to change the max vector geometry via a slider -> changing the amount of geometry drawn, would require us to fetch the data anew and preprocess them again. This requires that we clear what tiles have already been fetched.