png2rgba.py: Allow monochrome images
png2rgba.py: Allow monochrome images

DISTRHO/DPF,DISTRHO/DPF,DanielAeolusLaude/DPF,DanielAeolusLaude/DPF,DISTRHO/DPF,DISTRHO/DPF,DanielAeolusLaude/DPF,DanielAeolusLaude/DPF