Ashton post-processing does not respect Apple Retina displays

[danini-the-panini]

Running the ashton examples on my 2012 MacBook Pro Retina, the gosu window is created at four times the size (given 640 x 480, OS X gives gosu a 1280 x 960 viewport), however the Ashton post-processing still draws in the original 640 x 480 window. Below is a screenshot of the bloom example exhibiting this issue:

Is it possible to instruct Ashton to do post-processing over a larger area, or possibly detect the presence of a retina display and adjust accordingly?

[bil-bas]

I'm guessing that Gosu::Window#height and Gosu::Window#width returns the incorrect size, which is what Ashton uses to create the full-screen buffers it needs to perform post-processing.

See: gosu/gosu#244