Make SkCodec more flexible about its required frame

SkCodec sets fRequiredFrame to be the earliest possible frame that a
given frame can depend on. e.g.

- Frame A fills the screen, Keep
- Frame B does not cover A, Keep
- Frame C covers B but not A, and is opaque

Frame C can depend on either A or B. SkCodec already reports that C
depends on A. This CL allows a client of SkCodec to use either A or
B to create C.

Also expose the DisposalMethod. Since any frame between A and C can
be used to create C except for DisposePrevious frames, the client
needs to be able to know the disposal method so they do not try to
use such a frame to create C.

Further, the disposal method can be used to give the client a better
idea whether they will continue to need a frame. (e.g. if frame i is
DisposePrevious and depends on i-1, the client may not want to steal
i-1 to create i, since i+1 may also depend on i-1.)

TODO: Share code for decoding prior frames between GIF and WEBP

Change-Id: I91a5ae22ba3d8dfbe0bde833fa67ae3da0d81ed6
Reviewed-on: https://skia-review.googlesource.com/13722
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>

2 files changed, 0 insertions, 0 deletions

diff --git a/resources/required.gif b/resources/required.gif
new file mode 100644
index 0000000000..91a9fd17e8
--- /dev/null
+++ b/resources/required.gif
diff --git a/resources/required.webp b/resources/required.webp
new file mode 100644
index 0000000000..9f9a8f8b8d
--- /dev/null
+++ b/resources/required.webp