This removes some of the changes that meant we started using
mPermanentBackBuffer straight away and we now wait until we actually try and
lock a read locked texture.
While this might still give a very small risk of contention, it gives
improvements in the following two circumstances.
* If a canvas texture is never forwarded and never read locked, it means we will
only use one texture with no copies.
* If a canvas is always fully overwritten at the start of the frame (and a
snapshot is not taken between frames), then we avoid a copy on each frame.
This also adds back in code so that on an OPEN_READ_WRITE lock we cache the data
surface if required, because that texture will be the new front buffer and we
won't be using mPermanentBackBuffer at that point.
Depends on D132601
Differential Revision: https://phabricator.services.mozilla.com/D132602
a5aa570a4c