fix default window for native when no crop applied, https://github.com/hypertidy/lazyraster/issues/9
clarify DESCRIPTION thanks to CRAN feedback
crop() directly.S4 method crop() is now available for lazyraster.
Function as_raster() gains a native argument to override dimension.
Depend on vapour 0.4.0, because change in list return from read raster.
Depend on vapour 0.2.0.
Added a quadmesh method for lazyraster.