Initial version of alpha channel support

This is a mostly-working implementation of an extra channel in the
bitstream. Configure with --enable-alpha to test. Notable TODOs:

 - Add extra channel to all mismatch tests, PSNR, SSIM, etc
 - Configurable subsampling
 - Variable number of planes (currently always uses all 4)
 - Loop filtering
 - Per-plane lossless quantizer
 - ARNR support

This implementation just uses the same contents as the Y channel
for the A channel, due to lack of content and general pain in
playing back 4 channel content. A later patch will use the actual
alpha channel passed in from outside the codec.

Change-Id: Ibf81f023b1c570bd84b3064e9b4b8ae52e087592

1 files changed, 1 insertions, 0 deletions

diff --git a/configure b/configure
index cc8c58141..f4fea8d46 100755
--- a/configure
+++ b/configure
@@ -246,6 +246,7 @@ EXPERIMENT_LIST="
     oneshotq
     multiple_arf
     non420
+    alpha
     ab4x4
     comp_inter_joint_search
 "