Reset also viewport to default framebuffer on named FB destruction.

In text example the scene was rendered with viewport used for
distance field computation, which is wrong. Also amazingly enough that
worked until now.

src/Magnum/Framebuffer.cpp

@@ -28,6 +28,7 @@
 #include <Corrade/Containers/Array.h>
 
 #include "Magnum/Context.h"
+#include "Magnum/DefaultFramebuffer.h"
 #include "Magnum/Extensions.h"
 #include "Magnum/Image.h"
 #include "Magnum/Renderbuffer.h"
@@ -92,7 +93,17 @@ Framebuffer::~Framebuffer() {
     /* If bound, remove itself from state */
     Implementation::FramebufferState* state = Context::current()->state().framebuffer;
     if(state->readBinding == _id) state->readBinding = 0;
-    if(state->drawBinding == _id) state->drawBinding = 0;
+
+    /* For draw binding reset also viewport */
+    if(state->drawBinding == _id) {
+        state->drawBinding = 0;
+
+        /**
+         * @todo Less ugly solution (need to call setViewportInternal() to
+         * reset the viewport to size of default framebuffer)
+         */
+        defaultFramebuffer.bind(FramebufferTarget::Draw);
+    }
 
     glDeleteFramebuffers(1, &_id);
 }