Convert NNZ to raster order and other optimizations

Convert NNZ to raster order and other optimizations
Converting NNZ to raster order simplifies a lot of the load/store code and allows more use of write-combining.
More use of write-combining throughout load/save code in common/macroblock.c
GCC has aliasing issues in the case of stores to 8-bit heap-allocated arrays; dereferencing the pointer once avoids this problem and significantly increases performance.
More manual loop unrolling and such.
Move all packXtoY functions to macroblock.h so any function can use them.
Add pack8to32.
Minor optimizations to encoder/macroblock.c