Increase ability to load files > 200 MB on windows

[springmeyer]

TileMill 0.9.0 on windows could barely touch a raster without crashing: #1355. Now TileMill 0.9.1 on windows can load rasters up to around 200 MB, but larger still crashes. My hunch is the problem is the data copying: #1363 and #1354, whereby the cp command runs out of memory. The next task is, on windows 7, to try returning to using symlinks (#1504), and then to see if large (~ 1GB) rasters work, which they should. There is nothing on the Mapnik or GDAL side that cannot handle rasters this size, as long as OVERVIEWS are built with gdaladdo.

[springmeyer]

fixed by tilemill-project/millstone@74e9b45. just touching a file of 300 MB with node's fs.readFile would crash instantaneously on windows.