Add option to force a vector layer to render as a raster

[yjacolin]

qgis/QGIS@cfee456
Author: Nyall Dawson
Date: Thu Jul 9 18:34:55 2015 +1000

(under the layer properties, rendering tab)

So why would you want this? Well, extremely detailed layers (eg
polygon layers with a huge number of nodes) can cause composer
exports in PDF/SVG format to be huge as all nodes are included in
the exported file. This can also make the resultant file very slow
to work with/open in other programs (*cough* Inkscape *cough*).

Now, these you can force these layers to be rasterised so that the
exported files won't have to include all the nodes contained in these
layers. (Before you could also do this by forcing the composer to
export as a raster, but that was an all-or-nothing solution).

The ideal solution would be a simplification option for composer
exports which would simplify the layers by removing redundant points
at the export DPI, but this is an easy workaround for now.