asm: move cvt* functions to blockcopy8.asm
asm: remove more unused funcdefs from pixel.h
asm: move transpose from pixel-a.asm to pixel-util8.asm, add pixel-util.h
asm: move SSIM functions to pixel-util
asm: move scale functions to pixel-util
pixel: remove an unused macro
asm: move pixel_sub to pixel-util8.asm, move pixel_avg funcdef to mc.h
asm: move variance functions to pixel-util8.asm
asm: move ssd functions into their own ssd-a.asm file, similar to sad-a.asm
asm: make it more clear that pixel-a.asm has only satd and sa8d now