big image scalebar fix

[will-moore]

Bug fix: See https://trello.com/c/S78nkV0c/203-bug-scalebars-in-exported-pdf-big-images (reported at https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=8523&p=19515)

To test:

• Check that scalebars are drawn correctly at various zoom levels of a big image.
• I did this by drawing ROI lines of known length in iviewer, using the scalebar on the image.
• Then add these Lines as ROIs in figure and add scalebars of corresponding length, e.g. screenshot below:
• Add some scalebars to non-big images to check they still work OK.
• Export to PDF & TIFF and check that scalebars show up at same level as in web.

[rgozim]

Seems okay to me.

[jburel]

Having the unit only hard-coded in microns is far from ideal for big images. Adding a card since we should fix that https://trello.com/c/R2mWNfR9/205-scale-bar-unit