[Screencap] StitchImages optimized

[takahiro0327]

StitchImages has been optimized.
Please merge it if there are no problems.

The processing time was measured using the code below, and the processing time that used to take 4 seconds was reduced to 0.5 seconds. Depending on the capture parameters, it can be sped up by a factor of 5 to 8.

Stopwatch watch = Stopwatch.StartNew();
var result = FlipEyesIn3DCapture.Value ? StitchImages(capture, capture2, 0) : StitchImages(capture2, capture, 0);
System.Console.WriteLine($"@@ {watch.ElapsedMilliseconds}");

There is no visible change.
Before:

After:

I think it would be more than 10 times faster if the processing were completed entirely in VRAM, including the argument textures. However, additional VRAM would be required for the buffer.
I refrained from doing so because I knew that people would inevitably complain that they could no longer capture.

[ManlyMarco]

Very clean