Add missing files for GL unix viewer.

tools/viewer/sk_app/unix/GLWindowContext_unix.h

Index: tools/viewer/sk_app/unix/GLWindowContext_unix.h
diff --git a/tools/viewer/sk_app/win/GLWindowContext_win.h b/tools/viewer/sk_app/unix/GLWindowContext_unix.h
similarity index 56%
copy from tools/viewer/sk_app/win/GLWindowContext_win.h
copy to tools/viewer/sk_app/unix/GLWindowContext_unix.h
index f30a805e7234a4374216ce3f96643bb53f5c26a1..2d76254c21012d710557dae0c2f00aff56452305 100644
--- a/tools/viewer/sk_app/win/GLWindowContext_win.h
+++ b/tools/viewer/sk_app/unix/GLWindowContext_unix.h
@@ -5,19 +5,19 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
-#ifndef GLWindowContext_win_DEFINED
-#define GLWindowContext_win_DEFINED
+#ifndef GLWindowContext_unix_DEFINED
+#define GLWindowContext_unix_DEFINED
 
-#include <windows.h>
 #include "../GLWindowContext.h"
+#include "Window_unix.h"
 
 namespace sk_app {
 
-class GLWindowContext_win : public GLWindowContext {
+class GLWindowContext_unix : public GLWindowContext {
 public:
     friend GLWindowContext* GLWindowContext::Create(void* platformData, const DisplayParams&);
 
-    ~GLWindowContext_win() override;
+    ~GLWindowContext_unix() override;
 
     void onSwapBuffers() override;
 
@@ -25,10 +25,12 @@ public:
     void onDestroyContext() override;
 
 private:
-    GLWindowContext_win(void*, const DisplayParams&);
+    GLWindowContext_unix(void*, const DisplayParams&);
 
-    HWND              fHWND;
-    HGLRC             fHGLRC;
+    Display*     fDisplay;
+    XWindow      fWindow;
+    XVisualInfo* fVisualInfo;
+    GLXContext   fGLContext;
 };