Merge damage region when dropping an app buffer

When we decide to drop a buffer from a buffer queue, make sure to merge
the damage region from the dropped buffer into the current damage
region. Otherwise we'll pass the wrong damage region to hardware
composer and get broken rendering.

Bug: 136158117
Test: Wrote a new test in Transaction_test.cpp to repro the problem and
confirm the fix works.

Change-Id: Icdc61e1be3297450f2869c496dad1453fb6dca6d