Fix for var_tx entropy context with rect_tx
Fix for var_tx entropy context with rect_tx

This computation should match the code in encode_block
to increase the accuracy of the rd optimization.

Change-Id: Ibc9d9ab6d88d0c0f3af62e9cc233216aba48a57e

GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,luctrudeau/aom,smarter/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,luctrudeau/aom