Optimisations required for 1080p

[sipsorcery]

Attempting to receive a VP8 encoded video stream at 1080p (1920x1080) will quickly degrade to become very laggy and then corrupted. The sipsorcery library is not able to decode the video stream fast enough which results in the network buffer filling up and then dropping packets. The library is able to keep up with 720p (1280x720) on a Win10 i7 CPU.

The main bottleneck is the I420toBGR function.

[jvakos]

I don't have any experience in video enc/dec but in past I had to do some image processing (pixel by pixel processing). Back then, I gave a performance boost by using unsafe code block.

`BitmapData data = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height),
ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

int scaledPercent = percent * 255;
unsafe {
byte* prevLine = (byte*)data.Scan0;
byte* currLine = prevLine + data.Stride;
byte* nextLine = currLine + data.Stride;

for (int y=1; y < bitmap.Height - 1; y++) {

   byte* pp = prevLine + 3;
   byte* cp = currLine + 3;
   byte* np = nextLine + 3;
   for (int x = 1; x < bitmap.Width - 1; x++) {
       //here goes the proceccing
       pp += 3; cp += 3; np += 3;
   }
   prevLine = currLine;
   currLine = nextLine;
   nextLine += data.Stride;
}

}
`