Use non-default initial clear color in Renderer.

Set it to some non-trivial value to achieve these two things:

 * Some "corporate identity", I find very dark gray nicer than pure
   black.
 * Easier "black screen" debugging. Native Client, for example, by
   default clears to pure white, which caused me painful hours figuring
   out what the hell is going on.

src/Renderer.cpp

@@ -205,6 +205,9 @@ void Renderer::initializeContextBasedFunctionality(Context& context) {
     #else
     static_cast<void>(context);
     #endif
+
+    /* Set some "corporate identity" */
+    setClearColor(Color3(0.125f));
 }
 
 #ifndef MAGNUM_TARGET_GLES

src/Renderer.h

@@ -234,7 +234,7 @@ class MAGNUM_EXPORT Renderer {
         /**
          * @brief Set clear color
          *
-         * Initial value is fully opaque black.
+         * Initial value is {0.125f, 0.125f, 0.125f, 1.0f}.
          * @see @fn_gl{ClearColor}
          */
         static void setClearColor(const Color4& color);