Video backends: unify xfregs/xfmem structures.

Removes the duplicate swxfregs global variable/struct from the software
backend in favor of the ones from VideoCommon.
Video backend: merge global var xfmem into xfregs.

There isn't really any reason to keep them separate.
Video backends: mass-replace "xfregs" with "xfmem".
VideoCommon: cleanup Renderer::RecordVideoMemory.
Merge pull request #269 from magumagu/swbackend-xfregisters

SW backend: use VideoCommon XFRegisters struct.