surfaceflinger: Fix top-bottom UI composition with S3D.

-Fix the scissor computation for top and bottom frame in S3D.
-Fix the artifacts seen in top-bottom case by clearing the
 color buffer with scissor applied.

CRs-fixed: 346986
(cherry picked from commit 241b5b6b41fa4e1b2eca3565fd27b454a6aff262)

Change-Id: I8fb98007f67351009762c4494e260142b998ddb5