This commit is an optimization for double buffering that delays the creation of
a back buffer until we know what kind of content type it needs to be.
Before this commit, we would EnsureBackBufferIfFrontBuffer before BeginPaint,
then in BeginPaint we could determine that we actually needed a different kind
of buffer because the content changed type, and recreate it.
This was needed because BeginPaint would copy the old front buffer to the buffer
created by EnsureBackBufferIfFrontBuffer, and then if anything failed or we had
determined we couldn't reuse the buffer, we would create a new one and copy that
"temporary" back buffer over, and use the new one.
This is unnecessary because we only need read access on that "temporary" back
buffer, and so we can just use the current front buffer instead.
This optimization only affects the double buffered case, and the single buffered
or basic cases should remain the same.
Note: Because we now need the front buffer for copying into the new back buffer,
we cannot Clear() it away in some error cases.
Note: The code in FinalizeFrame assumes that the back and front buffer have the
same size. This was implicitly enforced before, and now needs to be explicitly
enforced. This commit tries to preserve the exact same behavior, although the
restriction should be removed as long as the back buffer is large enough for
the visible region.
MozReview-Commit-ID: 2hyrrUhA4zO
--HG--
extra : rebase_source : 5db62a4d24383c7d64e127880a3bee7ff76dada1
extra : intermediate-source : dce585be0737f3c9b6b241afb0851d85fb9453c9
extra : source : 926af2eca400cf8a16777813ceb586b1d3be7d68
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