Wrapper for drawing images with imlib2 and cairo

One problem is, that tolua/lua does not handle plain data pointers
properly. Therefore something like

local surface = cairo_image_surface_create_for_data(
    imlib_image_get_data_for_reading_only(), ..

will fail using the lua bindings. Hence this wrapper, which adds
a function which be used to draw an imlib image to a cairo surface.

This also addresses another issue, drawing images with a transparent
background (e.g. gifs or pngs) to the cairo surface properly. Apparently
the image format of imlib is incompatible to cairos which uses
pre-multiplied data. This is also explained in the cairo documentation.
A fix for this issue is to blend the image with another alpha mask to
get the correct values. The code and explanation for this was taken from

  http://lists.cairographics.org/archives/cairo/2008-March/013492.html

Executive Summary: This wrapper function can load (transparent) images
(e.g. gif or png) from a file by using imlib and draw them on a cairo
surface.
Merge pull request #14 from jrk-/image_wrapper

Wrapper for drawing images with imlib2 and cairo