Use privately Corrade Optional instead of the problematic std::optional.

Not in the public APIs yet (Trade etc.), as I need to solve backwards
compatibility first.

src/Magnum/Context.h

@@ -34,12 +34,12 @@
 #include <bitset>
 #include <vector>
 #include <Corrade/Containers/EnumSet.h>
+#include <Corrade/Containers/Optional.h>
 
 #include "Magnum/Magnum.h"
 #include "Magnum/OpenGL.h"
 #include "Magnum/Tags.h"
 #include "Magnum/visibility.h"
-#include "MagnumExternal/Optional/optional.hpp"
 
 namespace Magnum {
 
@@ -602,7 +602,7 @@ class MAGNUM_EXPORT Context {
 
         Implementation::State* _state;
 
-        std::optional<DetectedDrivers> _detectedDrivers;
+        Containers::Optional<DetectedDrivers> _detectedDrivers;
 
         /* True means known and disabled, false means known */
         std::vector<std::pair<std::string, bool>> _driverWorkarounds;

src/Magnum/Implementation/RendererState.cpp

@@ -105,7 +105,7 @@ RendererState::PixelStorage::PixelStorage():
 
 void RendererState::PixelStorage::reset() {
     #ifndef MAGNUM_TARGET_GLES
-    swapBytes = std::nullopt;
+    swapBytes = Containers::NullOpt;
     #endif
     alignment = DisengagedValue;
     #if !(defined(MAGNUM_TARGET_GLES2) && defined(MAGNUM_TARGET_WEBGL))

src/Magnum/Implementation/RendererState.h

@@ -27,10 +27,10 @@
 
 #include <string>
 #include <vector>
+#include <Corrade/Containers/Optional.h>
 
 #include "Magnum/Renderer.h"
 #include "Magnum/Math/Vector3.h"
-#include "MagnumExternal/Optional/optional.hpp"
 
 namespace Magnum { namespace Implementation {
 
@@ -52,7 +52,7 @@ struct RendererState {
         void reset();
 
         #ifndef MAGNUM_TARGET_GLES
-        std::optional<bool> swapBytes;
+        Containers::Optional<bool> swapBytes;
         #endif
         Int alignment;
         #if !(defined(MAGNUM_TARGET_GLES2) && defined(MAGNUM_TARGET_WEBGL))

src/Magnum/PixelStorage.cpp

@@ -220,7 +220,7 @@ void PixelStorage::applyInternal(const bool isUnpack) {
 
     #ifndef MAGNUM_TARGET_GLES
     /* Byte swap */
-    if(state.swapBytes == std::nullopt || state.swapBytes != _swapBytes)
+    if(state.swapBytes == Containers::NullOpt || state.swapBytes != _swapBytes)
         glPixelStorei(isUnpack ? GL_UNPACK_SWAP_BYTES : GL_PACK_SWAP_BYTES,
           *(state.swapBytes = _swapBytes));
     #endif

src/Magnum/Shaders/MeshVisualizer.cpp

@@ -25,12 +25,12 @@
 
 #include "MeshVisualizer.h"
 
+#include <Corrade/Containers/Optional.h>
 #include <Corrade/Utility/Resource.h>
 
 #include "Magnum/Context.h"
 #include "Magnum/Extensions.h"
 #include "Magnum/Shader.h"
-#include "MagnumExternal/Optional/optional.hpp"
 
 #include "Implementation/CreateCompatibilityShader.h"
 
@@ -86,7 +86,7 @@ MeshVisualizer::MeshVisualizer(const Flags flags): _flags{flags} {
         .addSource(rs.get("MeshVisualizer.frag"));
 
     #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
-    std::optional<Shader> geom;
+    Containers::Optional<Shader> geom;
     if(flags & Flag::Wireframe && !(flags & Flag::NoGeometryShader)) {
         geom = Implementation::createCompatibilityShader(rs, version, Shader::Type::Geometry);
         geom->addSource(rs.get("MeshVisualizer.geom"));