PCE: 50% overall speedup by reworking BG render inner loop

git-svn-id: http://bizhawk.googlecode.com/svn/trunk@6102 4bbf4f03-6f44-549e-7e59-35d4b0962838
apply r6102 to SGX.  60% speedup in that arthur game

git-svn-id: http://bizhawk.googlecode.com/svn/trunk@6103 4bbf4f03-6f44-549e-7e59-35d4b0962838