Force intermediary PNGs to be 24 bit
Force intermediary PNGs to be 24 bit

If the source PDF is black and white, ImageMagick will try to convert to
grayscale, but then it seems to complain that the RGB color space (I
guess from the source) is not permitted on grayscale PNG.

Forcing PNG24 avoids the grayscale conversion, and thus the warning.

cgunther/rspec-pdf_diff