Set the default background color to something recognisable.

src/video.cpp

@@ -77,7 +77,7 @@ void Video::Setup(int width, int height)
 #if defined HAVE_GL_2X || defined HAVE_GLES_1X
     glShadeModel(GL_SMOOTH);
 #endif
-    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
+    glClearColor(0.1f, 0.2f, 0.3f, 0.0f);
     glClearDepthf(1.0);
 
 #if defined HAVE_GL_2X || defined HAVE_GLES_1X