PSD: remove hardcoded 32 planes
fix/workaround for associating alpha for 8bit photoshop files

This should be fixed upstream, and it may not be the correct formula.
I'm debating over the commited and the following:

static void photoshop_associate_alpha(uchar * pixels, int width, int
height, uchar background[4])
{
    uchar alpha;
    for (int i = 0; i < width*height; i++) {
        alpha = pixels[i*4+3];

        pixels[i*4+2] -= background[0] - alpha;
        pixels[i*4+1] -= background[1] - alpha;
        pixels[i*4+0] -= background[2] - alpha;
    }
}

The formulas are only different if/when background is not 0xFFFFFF.