Drawing to render texture in EditorWindow

Hello.

I'm trying to make visual editor for my GLUe Framework (https://www.assetstore.unity3d.com/#/content/6265), so i need to display some meshes via GL and Graphics in the editor window.

When i using:

 public void OnGUI()
 {
      GL.PushMatrix();
      GL.LoadPixelMatrix(0, Screen.width, 0, Screen.height);
 
      // Draw my stuff
 
      GL.PopMatrix();
 }

everything works fine, but when i add render texture:

 public void OnGUI()
 {
     GL.PushMatrix();
     GL.LoadPixelMatrix(0, Screen.width, 0, Screen.height);
     
     // Draw my stuff
 
     GL.PushMatrix();
     RenderTexture rt = RenderTexture.active;
     RenderTexture.active = mytexture;
     GL.LoadPixelMatrix(0, mytexture.width, 0, mytexture.height);
     
     // Draw stuff to render texture

     RenderTexture.active = rt;
     GL.PopMatrix();
 
     // Assign render texture to material
     // Assign material to polygon and draw it

     GL.PopMatrix();
 }

RT contents stretches not like other stuff. It look like GL matrix is not restored properly after last Pop. Has anyone encountered such thing before?
PS: In run-time both things work fine (although, i use OnPostRender() instead of OnGUI() ).

UPD: The trouble is exactly with render texture - when i remove "RenderTexture.active = mytexture;", the rest of form is drawn without stretching.