title: GetPixels and SetPixels for only alpha channel
body: <p>I want to change alpha-channel of RenderedTexture, that is target of camera. I know, I can GetPixels from Texture2d and SetPixels to another Texture2d. But how can I copy pixels from my own .psd texture (I need only alpha-channel pixels) and pass it into RenderTexture?</p>
  
  <p>UPD: I can ReadPixels to temp texture from RenderTexture, but how can I GetPixels and SetPixels for only one channel?</p>