gallium: Refactor pack/unpack calls

I had an idea for how to shrink the built size of the util/format/ pack/unpack code (make the implementations be for rows, not rectangles), and started refactoring in that direction. It didn't pan out because softpipe wants to load rects full of compressed texture data, but in the process I deleted 500 lines of code and 20k of binary, so I'm considering it a day well spent.