Replay buffer commands on paint thread when OMTP is enabled (bug 1399692 part 7, r=bas)

This commit does the work of actually dispatching the recorded buffer operations
to the paint thread, and removing some main thread asserts from TextureClient.

MozReview-Commit-ID: CN3RoQPz9fP

gfx/layers/client/TextureClient.cpp

@@ -692,10 +692,6 @@ TextureClient::BorrowDrawTarget()
     return nullptr;
   }
 
-  if (!NS_IsMainThread()) {
-    return nullptr;
-  }
-
   if (!mBorrowedDrawTarget) {
     mBorrowedDrawTarget = mData->BorrowDrawTarget();
 #ifdef DEBUG