GL: don't reallocate std::string in DebugOutput

src/Magnum/GL/DebugOutput.cpp

@@ -104,8 +104,10 @@ void
 APIENTRY
 #endif
 callbackWrapper(GLenum source, GLenum type, GLuint id, GLenum severity, GLsizei length, const GLchar* message, const void* userParam) {
-    const auto& callback = *static_cast<const Implementation::DebugState::MessageCallback*>(userParam);
-    callback.callback(DebugOutput::Source(source), DebugOutput::Type(type), id, DebugOutput::Severity(severity), std::string{message, std::size_t(length)}, callback.userParam);
+    auto& callback = *static_cast<Implementation::DebugState::MessageCallback*>(const_cast<void*>(userParam));
+    callback.buf.reserve(256);
+    callback.buf.assign(message, std::size_t(length));
+    callback.callback(DebugOutput::Source(source), DebugOutput::Type(type), id, DebugOutput::Severity(severity), callback.buf, callback.userParam);
 }
 
 }

src/Magnum/GL/Implementation/DebugState.h

@@ -50,6 +50,7 @@ struct DebugState {
     struct MessageCallback {
         DebugOutput::Callback callback{};
         const void* userParam{};
+        std::string buf;
     } messageCallback;
 };