mrcolour: png upside down

[maxpietsch]

mrcolour produces pngs that are upside down. macOS, PNG library: Header: 1.6.37; library: 1.6.37

wget https://github.com/MRtrix3/script_test_data/raw/master/BIDS/sub-02/dwi/sub-02_dwi.nii.gz
mrconvert sub-02_dwi.nii.gz -coord 3 0 -coord 0 15  sag.mif
mrcolour sag.mif gray sag.png

mrconvert sub-02_dwi.nii.gz -coord 3 0 -coord 1 15  cor.mif
mrcolour cor.mif gray cor.png

[Lestropie]

Applies to the PNG format generally, not specifically to mrcolour.

I spent some time getting this right, so not quite sure what happened at the tail end of #1541 to send it skewiff again.

[Lestropie]

@jdtournier suggested that user stride request could be used to attribute image axes to 2D PNG axes. This might be possible. I need to essentially re-write the relevant section of code as it is to try to address this, so I'll try to keep this prospect in mind.

[Lestropie]

Closing as issue became part of the expanded scope of #2713.