Bug 963917: reduce Gallery memory use (and fix leak) while scanning

force software canvases for thumbnail and preview creation with willReadFrequently

Free canvas memory after creating a thumbnail

Make sure our thumbnail blobs are file-backed not memory-backed

update mediadb docs
Merge pull request #16148 from davidflanagan/bug963917-v1.3-3

Bug 963917: reduce Gallery memory use (and fix leak) while scanning r=dkuo