Don't use zero-sized viewport as disengaged state.

Zero-sized viewport is perfectly valid state and it causes assertion
failures on contexts without default framebuffer (e.g.
WindowlessCglApplication).

src/Magnum/Implementation/FramebufferState.h

@@ -33,7 +33,7 @@
 namespace Magnum { namespace Implementation {
 
 struct FramebufferState {
-    constexpr static const Range2Di DisengagedViewport{};
+    constexpr static const Range2Di DisengagedViewport{{}, {-1, -1}};
 
     explicit FramebufferState(Context& context, std::vector<std::string>& extensions);