Modify WebcamTexture from unity native plugin - unity3d

Question

I just want to pass a WebcamTexture to a native plugin (using GetNativeTexturePtr() for performance), draw something on it and render results into a plane on the screen.
I guess that some Unity thread would be updating the WebcamTexture contents every frame so writing to that texture may produce some flickering effects as there would be 2 threads updating its contents. Because of that I'm using another texture "drawTexture" to render the result.
The algorithm is easy, on every render event:
Read camTexture
Copy camTexture contents to drawTexture
Draw things on drawTexture
Render drawTexture (OpenGL bindTexture)
I'm following the NativeRenderingPlugin example, but every time I try to copy contents from camTexture to drawTexture (even only 1 pixel) the main thread freezes.
I think that may be happening because I'm trying to read camTexture while it is being modified by an external thread.
Here it is some code:
C# plugin source
[DllImport ("plugin")]
private static extern void setTexture (IntPtr cam, IntPtr draw, int width, int height);
[DllImport ("plugin")]
private static extern IntPtr GetRenderEventFunc();
WebCamTexture webcamTexture;
Texture2D drawTexture;
GameObject plane;
bool initialized = false;
IEnumerator Start () {
// PlaneTexture
// Create a texture
drawTexture = new Texture2D(1280,720,TextureFormat.BGRA32,true);
drawTexture.filterMode = FilterMode.Point;
drawTexture.Apply ();
plane = GameObject.Find("Plane");
plane.GetComponent<MeshRenderer> ().material.mainTexture = drawTexture;
// Webcam texture
webcamTexture = new WebCamTexture();
webcamTexture.filterMode = FilterMode.Point;
webcamTexture.Play();
yield return StartCoroutine("CallPluginAtEndOfFrames");
}
private IEnumerator CallPluginAtEndOfFrames()
{
while (true) {
// Wait until all frame rendering is done
yield return new WaitForEndOfFrame();
// wait for webcam and initialize
if (!initialized && webcamTexture.width > 16) {
setTexture (webcamTexture.GetNativeTexturePtr(), drawTexture.GetNativeTexturePtr(), webcamTexture.width, webcamTexture.height);
initialized = true;
} else if (initialized) {
// Issue a plugin event with arbitrary integer identifier.
// The plugin can distinguish between different
// things it needs to do based on this ID.
// For our simple plugin, it does not matter which ID we pass here.
GL.IssuePluginEvent (GetRenderEventFunc (), 1);
}
}
}
C++ plugin (draw texture method)
static void ModifyTexturePixels()
{
void* textureHandle = drawTexturePointer;
int width = textureWidth;
int height = textureHeight;
if (!textureHandle)
return;
int textureRowPitch;
void* newEmptyTexturePointer = (unsigned char*)s_CurrentAPI->BeginModifyTexture(textureHandle, width, height, &textureRowPitch);
memcpy(newEmptyTexturePointer, webcamTexturePointer, width * height * 4);
s_CurrentAPI->EndModifyTexture(textureHandle, width, height, textureRowPitch, newEmptyTexturePointer);
}
Any help would be appreciated.