Raster rendering optimizations

[knutole]

Currently, it seems all tiles (even if empty), take same amount of time to render.

From console.time:

create cube tile: 2410ms
create cube tile: 2331ms
create cube tile: 2806ms
create cube tile: 2843ms
create cube tile: 2849ms
create cube tile: 2526ms
create cube tile: 2823ms
create cube tile: 2785ms
create cube tile: 2053ms
create cube tile: 2592ms
create cube tile: 2697ms
create cube tile: 2748ms
create cube tile: 2745ms
create cube tile: 2707ms
create cube tile: 1845ms
create cube tile: 2201ms
create cube tile: 2784ms
create cube tile: 2161ms
create cube tile: 2180ms
create cube tile: 2435ms

Optimizations

• Detect if requested tile is outside extent of raster. If so, don't render, but simply return empty tile immediately.