Fix scaling of 1bit grayscale pixels in renderDeviceGrayToPNG

[ungerik]

In renderDeviceGrayToPNG color.Gray.Y is expected to be in the range of 0 .. 255, but the default was colValRange{0, 1}.

I have also removed the unnecessary allocation of a []colValRange slice per pixel which should improve performance.

[hhrutter]

There are 2 separate steps involved here:

1. Decoding the pixel value into the source color space respecting the optional DecodeArray
2. Scaling up to 8 bpc

Thank you for your contribution!