avoid layout when thumbnails load

[bwindels]

set the thumbnail dimensions from the event on the before the image loads in MImageBody, so they don't resize when the image is loaded.

[bwindels]

#8565 should have already improved this for Chrome, but as this is probably the biggest cause for scroll jumping, seems like a no-brainer to do this for other browsers as well.

[MadLittleMods]

Related to #9670

[t3chguy]

Duplicate of #9670