Copy macroblock data to a buffer before encoding it

Copy macroblock data to a buffer before encoding it

I got this idea from Pascal (Thanks). Before encoding a macroblock,
copy it to a 16x16 buffer, and then read source data from there
instead. This will help keep the source data in cache, and help
with the performance.

Change-Id: Id05f4cb601299150511d59dcba0ae62c49b5b757