Use Windowing system-specific WindowContext factories.

tools/viewer/sk_app/unix/WindowContextFactory_unix.h

+
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef WindowContextFactory_unix_DEFINED
+#define WindowContextFactory_unix_DEFINED
+
+#include <X11/Xlib.h>
+#include <GL/glx.h>
+typedef Window XWindow;
+
+namespace sk_app {
+
+class WindowContext;
+struct DisplayParams;
+
+namespace window_context_factory {
+
+struct XlibWindowInfo {
+    Display*     fDisplay;
+    XWindow      fWindow;
+    XVisualInfo* fVisualInfo;
+};
+
+WindowContext* NewVulkanForXlib(const XlibWindowInfo&, const DisplayParams&);
+
+WindowContext* NewGLForXlib(const XlibWindowInfo&, const DisplayParams&);
+
+}  // namespace window_context_factory
+
+}  // namespace sk_app
+
+#endif