Add support for blending (composition) modes, eg Overlay, Mutiply, etc (#5248)

Adds support for blending modes for vector and raster layers. Allows for some fancy cartographic effects (along the lines of http://mapbox.com/blog/tilemill-compositing-operations-preview/)!

Works for all vector and raster layers, exporting maps and composers as images, and for exporting composers as pdfs IF "print as raster" is checked. 

Todo: investigate if compositing is possible for standard pdf output.