make image.im3d useRaster when possible

* but see https://github.com/jefferis/nat/issues/39 for discussion of why
  this doesn't work as it should
image.im3d will use plot images as rasters where possible

* noted 20x speed for medium size images
* closes #39 by modifying a few lines of image.default rather than deferring to
  image.default's (currently broken) logic for determining if rasterImage can
  be used.
Bump version to 1.4.4