imwrite for double grayvalues fails (regression)

[Andreas Weber]

Dear maintainers,
after changeset b76b14e386b3
(http://hg.savannah.gnu.org/hgweb/octave/rev/b76b14e386b3?revcount=120)

imwrite with grayvalues in the range 0..1 produces a black image.

For example

a=repmat(linspace(0,1,200),100,1);
imwrite(a, test.png)

should give a image wich fades horizontally from black to white.
This is because (1 << bitdepth) gives an int overflow for bitdepth=32.

Should we use a long int:
const double divisor = static_cast<double>((1L << bitdepth) - 1) / MaxRGB;

or is there a better fix?
Andy