Audio: make Context only move-constructible.

For some reason it was copyable and everything. Also fix the cleanup on
destruction so it can be created more than once.

src/Magnum/Audio/Context.cpp

@@ -155,11 +155,16 @@ Context::Context(const Configuration& config) {
     Debug() << "OpenAL version:" << versionString();
 }
 
-Context::~Context() {
-    CORRADE_INTERNAL_ASSERT(_current == this);
+Context::Context(Context&& other) noexcept: _device{other._device}, _context{other._context}, _extensionStatus{std::move(other._extensionStatus)}, _supportedExtensions{std::move(other._supportedExtensions)} {
+    other._device = nullptr;
+    other._context = nullptr;
+    if(_current == &other) _current = this;
+}
 
-    alcDestroyContext(_context);
-    alcCloseDevice(_device);
+Context::~Context() {
+    if(_context) alcDestroyContext(_context);
+    if(_device) alcCloseDevice(_device);
+    if(_current == this) _current = nullptr;
 }
 
 std::vector<std::string> Context::extensionStrings() const {

src/Magnum/Audio/Context.h

@@ -169,6 +169,12 @@ class MAGNUM_AUDIO_EXPORT Context {
         explicit Context();
         #endif
 
+        /** @brief Copying is not allowed */
+        Context(const Context&) = delete;
+
+        /** @brief Move constructor */
+        Context(Context&& other) noexcept;
+
         /**
          * @brief Destructor
          *
@@ -176,6 +182,12 @@ class MAGNUM_AUDIO_EXPORT Context {
          */
         ~Context();
 
+        /** @brief Copying is not allowed */
+        Context& operator=(const Context&) = delete;
+
+        /** @brief Move assignment is not allowed */
+        Context& operator=(Context&& other) = delete;
+
         #if defined(MAGNUM_BUILD_DEPRECATED) && !defined(DOXYGEN_GENERATING_OUTPUT)
         CORRADE_DEPRECATED("Audio::Context::current() returns reference now") Context* operator->() { return this; }
         CORRADE_DEPRECATED("Audio::Context::current() returns reference now") operator Context*() { return this; }

src/Magnum/Audio/Test/ContextALTest.cpp

@@ -34,15 +34,45 @@ namespace Magnum { namespace Audio { namespace Test {
 struct ContextALTest: TestSuite::Tester {
     explicit ContextALTest();
 
+    void construct();
+    void constructMove();
+
     void extensionsString();
     void isExtensionEnabled();
 };
 
 ContextALTest::ContextALTest() {
-    addTests({&ContextALTest::extensionsString,
+    addTests({&ContextALTest::construct,
+              &ContextALTest::constructMove,
+
+              &ContextALTest::extensionsString,
               &ContextALTest::isExtensionEnabled});
 }
 
+void ContextALTest::construct() {
+    CORRADE_VERIFY(!Context::hasCurrent());
+
+    {
+        Context context;
+        CORRADE_VERIFY(Context::hasCurrent());
+        CORRADE_COMPARE(&Context::current(), &context);
+    }
+
+    CORRADE_VERIFY(!Context::hasCurrent());
+}
+
+void ContextALTest::constructMove() {
+    Context context;
+    CORRADE_COMPARE(&Context::current(), &context);
+
+    {
+        Context second{std::move(context)};
+        CORRADE_COMPARE(&Context::current(), &second);
+    }
+
+    CORRADE_VERIFY(!Context::hasCurrent());
+}
+
 void ContextALTest::extensionsString() {
     Context context;
 

src/Magnum/Audio/Test/ContextTest.cpp

@@ -34,17 +34,29 @@ namespace Magnum { namespace Audio { namespace Test {
 struct ContextTest: TestSuite::Tester {
     explicit ContextTest();
 
+    void constructCopyMove();
+
     void extensions();
 
     void debugHrtfStatus();
 };
 
 ContextTest::ContextTest() {
-    addTests({&ContextTest::extensions,
+    addTests({&ContextTest::constructCopyMove,
+
+              &ContextTest::extensions,
 
               &ContextTest::debugHrtfStatus});
 }
 
+void ContextTest::constructCopyMove() {
+    /* Only move-construction allowed */
+    CORRADE_VERIFY(!(std::is_constructible<Context, const Context&>{}));
+    CORRADE_VERIFY((std::is_constructible<Context, Context&&>{}));
+    CORRADE_VERIFY(!(std::is_assignable<Context, const Context&>{}));
+    CORRADE_VERIFY(!(std::is_assignable<Context, Context&&>{}));
+}
+
 void ContextTest::extensions() {
     const char* used[Implementation::ExtensionCount]{};