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
(cherry picked from commit 4b5eaa4a2823d8961b80a3b975b1eeed92b9fcb7)

Conflicts:

	apps/gallery/js/MetadataParser.js
	apps/gallery/js/gallery.js
Bug 930563 - [B2G][L10n][Clock] Chinese Traditional: Highlight date instead of first number displayed.
Bug 960705 - [clock][l10n] Close button should say Stop instead; r=reviews
Bug 969587 - Make customization run without ICCHelper present
Bug 969587 - Integration tests for FTU variant customization
Merge pull request #16212 from gnaneswar/Issue_960705

Bug 960705 - [clock][l10n] Close button should say Stop instead; r=mcav
Merge pull request #16186 from mcav/bold-date

Bug 930563 - [B2G][L10n][Clock] Chinese Traditional: Highlight date. r=gnaneswar
Merge pull request #16149 from davidflanagan/bug963917-3

Bug 963917: reduce Gallery memory use (and fix leak) while scanning r=dkuo
Merge pull request #16097 from qdot/969587-variant-with-no-icc

Bug 969587 - Run variant customization with no icc
Bug 918400 - [Clock] Vibrate should use a toggle instead of a drop down