Ability to move-construct Context.

Useful in this case:

    std::optional<Platform::Context> context;
    context = Platform::Context{};

This was currently possible only with calling context.emplace(), which
in my opinion is confusing and looks like a no-op. Move assignment is
still not allowed, because having two separate state trackers (and
deleting one of them) currently makes no sense.

src/Magnum/Context.cpp

@@ -543,10 +543,24 @@ Context::Context(void functionLoader()) {
     Renderer::initializeContextBasedFunctionality();
 }
 
+Context::Context(Context&& other): _version{std::move(other._version)},
+    #ifndef MAGNUM_TARGET_WEBGL
+    _flags{std::move(other._flags)},
+    #endif
+    _extensionRequiredVersion{std::move(other._extensionRequiredVersion)},
+    _extensionStatus{std::move(other._extensionStatus)},
+    _supportedExtensions{std::move(other._supportedExtensions)},
+    _state{std::move(other._state)},
+    _detectedDrivers{std::move(other._detectedDrivers)}
+{
+    other._state = nullptr;
+    if(_current == &other) _current = this;
+}
+
 Context::~Context() {
-    CORRADE_ASSERT(_current == this, "Context: Cannot destroy context which is not currently active", );
     delete _state;
-    _current = nullptr;
+
+    if(_current == this) _current = nullptr;
 }
 
 std::vector<std::string> Context::shadingLanguageVersionStrings() const {

src/Magnum/Context.h

@@ -210,15 +210,15 @@ class MAGNUM_EXPORT Context {
         /** @brief Copying is not allowed */
         Context(const Context&) = delete;
 
-        /** @brief Moving is not allowed */
-        Context(Context&&) = delete;
+        /** @brief Move constructor */
+        Context(Context&& other);
 
         ~Context();
 
         /** @brief Copying is not allowed */
         Context& operator=(const Context&) = delete;
 
-        /** @brief Moving is not allowed */
+        /** @brief Move assignment is not allowed */
         Context& operator=(Context&&) = delete;
 
         /** @brief Current context */