GL: drop another std::pair occurence from the internals.

Hm, and here I used the "capability" of std::pair that allowed it to
store references. I don't even want to know what all was involved to
support that, Containers::Reference is much easier to reason about.

src/Magnum/GL/Context.cpp

@@ -28,6 +28,7 @@
 #include <algorithm> /* std::lower_bound() */
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/GrowableArray.h>
+#include <Corrade/Containers/Reference.h>
 #include <Corrade/Containers/StringIterable.h>
 #include <Corrade/Utility/Arguments.h>
 #include <Corrade/Utility/Debug.h>
@@ -985,9 +986,9 @@ bool Context::tryCreate(const Configuration& configuration) {
             Debug{output} << "   " << extension.string();
     }
 
-    std::pair<Containers::ArrayTuple, Implementation::State&> state = Implementation::State::allocate(*this, output);
-    _stateData = std::move(state.first);
-    _state = &state.second;
+    Containers::Pair<Containers::ArrayTuple, Containers::Reference<Implementation::State>> state = Implementation::State::allocate(*this, output);
+    _stateData = std::move(state.first());
+    _state = &*state.second();
 
     /* Print a list of used workarounds */
     if(!_driverWorkarounds.isEmpty()) {

src/Magnum/GL/Implementation/State.cpp

@@ -27,6 +27,7 @@
 
 #include <Corrade/Containers/ArrayTuple.h>
 #include <Corrade/Containers/Pair.h>
+#include <Corrade/Containers/Reference.h>
 #include <Corrade/Utility/Assert.h>
 
 #include "Magnum/GL/Context.h"
@@ -49,7 +50,7 @@
 
 namespace Magnum { namespace GL { namespace Implementation {
 
-std::pair<Containers::ArrayTuple, State&> State::allocate(Context& context, std::ostream* const out) {
+Containers::Pair<Containers::ArrayTuple, Containers::Reference<State>> State::allocate(Context& context, std::ostream* const out) {
     /* TextureState needs to track state per texture / image binding, fetch
        how many of them is there and allocate here as well so we don't need to
        do another nested allocation */

src/Magnum/GL/Implementation/State.h

@@ -26,7 +26,6 @@
 */
 
 #include <iosfwd>
-#include <utility>
 #include <Corrade/Containers/Containers.h>
 
 #include "Magnum/Magnum.h"
@@ -53,7 +52,7 @@ struct TransformFeedbackState;
 struct State {
     /* Initializes context-based functionality together with all nested classes
        in a single allocation */
-    static std::pair<Containers::ArrayTuple, State&> allocate(Context& context, std::ostream* out);
+    static Containers::Pair<Containers::ArrayTuple, Containers::Reference<State>> allocate(Context& context, std::ostream* out);
 
     enum: GLuint { DisengagedBinding = ~0u };