Add geom_raster and geom_hexbin

[Vindaar]

geom_raster would be handy for heatmaps with many tiles. The current approach of drawing a single rectangle for each tile is extremely wasteful for large heatmaps, which essentially represent bitmaps.

We should attempt to utilize cairo rastering to implement this. As far as I understand it, cairo raster gets two callbacks to determine the color for each pixel.

Essentially we provide a proc, which calculates the color of each pixel in the main plot viewport in (I assume) global coordinates.

If this works out, we can think about implementing geom_hexbin as well. However, in order to support hex binning two dimensional data, we need a histogram proc for hex bins. But essentially if the hex size is known the computation of that is rather simple.

[Vindaar]

geom_raster has been implemented in #84.

What still is broken is using geom_raster with bin / count statistics. This is almost a requisite for geom_hexbin, because I assume almost no one would calculate a hex bin statistics manually. Drawing the hex bin is reasonably easy. Might be a good idea at that point to move some of the raster logic from ginger back to ggplotnim, so that a ginger backend only has to draw a finished bitmap without and logic attached.