…allest extent is 32 pixels or less. r=gw

The code that picks the texture cache slab size does not consider snapped extents under 64 pixels for rectangular pages, causing for example a 16x500 request to go into a 512x512 page.
This commit allows very thin requests to get a 64x512 page.

Differential Revision: https://phabricator.services.mozilla.com/D66187

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/03f0fbdc04416190d45e13e9aa67d9895db8240e