I wrote this very naive NEON implementation to convert from RGBA to RGB. It works but I was wondering if there was anything else I could do to further improve performances.

I tried playing around with the prefetching size and unrolling the loop a bit more but performances didn't change much. By the way, are there any rule of thumbs when it comes to dimension the prefetching? I couldn't find anything useful on the net. Furthermore in the "ARMv8 Instruction Set Overview" I see there's also a prefetch for store, how is that useful?

Currently I'm getting around 1.7ms to convert a 1280x720 image on an iPhone5s.

```
// unsigned int * rgba2rgb_neon(unsigned int * pDst, unsigned int * pSrc, unsigned int count);
_rgba2rgb_neon:
    cmp     w2, #0x7
    b.gt    loop

    mov     w0, #0
    ret

loop:
    prfm    pldl1strm, [w1, #64]

    ld4.8b  {v0, v1, v2, v3}, [w1], #32
    ld4.8b  {v4, v5, v6, v7}, [w1], #32

    prfm    pldl1strm, [w1, #64]

    st3.8b  {v0, v1, v2}, [w0], #24
    st3.8b  {v4, v5, v6}, [w0], #24

    subs    w2, w2, #16
    b.gt    loop

done:
    ret

```