基本思路：mat转rgba，转CGImageRef，再转到PixelBuffer

static void to_argb(const ncnn::Mat& m, u_int8_t* argb)
{
    const float* ptr0 = m.channel(0);
    const float* ptr1 = m.channel(1);
    const float* ptr2 = m.channel(2);
    
#define SATURATE_CAST_UCHAR(X) (u_int8_t)::std::min(::std::max((int)(X), 0), 255);
    int size = m.w * m.h;
    int remain = size;
    for (; remain>0; remain--)
    {
        argb[3]=255;
        argb[0] = SATURATE_CAST_UCHAR(*ptr0);//r
        argb[1] = SATURATE_CAST_UCHAR(*ptr1);//g
        argb[2] = SATURATE_CAST_UCHAR(*ptr2);//b

        argb += 4;
        ptr0++;
        ptr1++;
        ptr2++;
    }
#undef SATURATE_CAST_UCHAR
}

CVPixelBufferRef copyDataFromMat(const ncnn::Mat& m)
{
    u_int8_t *newBitmap = new u_int8_t[m.h*m.w*4];
    to_argb(m, newBitmap);
    //创建image
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    void *colorData = newBitmap;
    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, colorData, m.h*4*m.w, NULL);
    CGImageRef cgImage2 = CGImageCreate(m.w,
                                        m.h,
                                        8,
                                        8 * 4,
                                        m.w*4,
                                        colorSpace,
                                        kCGImageAlphaPremultipliedLast|kCGBitmapByteOrderDefault,
                                        provider,
                                        NULL,
                                        NO,
                                        kCGRenderingIntentDefault);
    CGDataProviderRelease(provider);
    UIImage *image = [UIImage imageWithCGImage:cgImage2];
    //转为PixelBuffer
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                             [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey,
                             [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey,
                             nil];
    CVPixelBufferRef pxbuffer = NULL;
    CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault,
                                          m.w,
                                          m.h,
                                          kCVPixelFormatType_32ARGB,
                                          (__bridge CFDictionaryRef) options,
                                          &pxbuffer);
    
    CVPixelBufferLockBaseAddress(pxbuffer, 0);
    void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer);
    CGContextRef context = CGBitmapContextCreate(pxdata,
                                                 m.w,
                                                 m.h,
                                                 8,
                                                 CVPixelBufferGetBytesPerRow(pxbuffer),
                                                 colorSpace,
                                                 (CGBitmapInfo)kCGImageAlphaNoneSkipFirst);
    CGContextConcatCTM(context, CGAffineTransformIdentity);
    CGContextDrawImage(context, CGRectMake(0,0,m.w,m.h),cgImage2);
    
    CGContextRelease(context);
    CGImageRelease(cgImage2);
    CGColorSpaceRelease(colorSpace);
    CVPixelBufferUnlockBaseAddress(pxbuffer, 0);
    delete[] newBitmap;
    return pxbuffer;
}

ios RGBA数据转UIImage