使用RenderTexture和camera.targetTexture生成快照

Camera有一個targetTexture屬性，在設置了targetTexture后，Camera會在渲染時將其屏幕上的圖像渲染到targetTexture上，可以使用快照實現鏡頭切換等特效

具體實現:

private Texture2D CaptureCamera(Camera camera, Rect rect)
{
RenderTexture buffer = RenderTexture.GetTemporary((int)rect.width, (int)rect.height, 1);
camera.targetTexture = buffer;
camera.Render();
// 激活這個rt, 並從中中讀取像素。
RenderTexture.active = buffer;
Texture2D screenShot = new Texture2D((int)rect.width, (int)rect.height, TextureFormat.RGB24, false);
screenShot.ReadPixels(rect, 0, 0);// 注：這個時候，它是從RenderTexture.active中讀取像素
screenShot.Apply();
// 重置相關參數，以使用 camera 繼續在屏幕上顯示
camera.targetTexture = null;
RenderTexture.active = null;
GameObject.Destroy(buffer);
return screenShot;
}