Query raster layers

[jelmervdl]

I'm working a bit with raster layers in PostgreSQL and it would be kind of cool if we could also show those layers in the actual map.

Easiest implementation would be via data urls for transport in the query output and ST_AsPNG in Postgis, but that won't be really scaleable. Being able to zoom out & in and get a resampled or more detailed visualisation would be way cooler.