GL: deduplicate & simplify DebugOutput::setCallback() internals.

The previous callback pointer was needed just to disambiguate, so it can
be a bool; the user pointer can be set from the caller already instead
of being done in each variant again.

src/Magnum/GL/DebugOutput.cpp

@@ -145,7 +145,8 @@ Int DebugOutput::maxMessageLength() {
 }
 
 void DebugOutput::setCallback(const Callback callback, const void* userParam) {
-    Context::current().state().debug.callbackImplementation(callback, userParam);
+    Context::current().state().debug.messageCallback.userParam = userParam;
+    Context::current().state().debug.callbackImplementation(callback);
 }
 
 void DebugOutput::setDefaultCallback() {
@@ -172,38 +173,38 @@ void DebugOutput::controlImplementationKhrES(const GLenum source, const GLenum t
 }
 #endif
 
-void DebugOutput::callbackImplementationNoOp(Callback, const void*) {}
+void DebugOutput::callbackImplementationNoOp(Callback) {}
 
 #ifndef MAGNUM_TARGET_GLES2
-void DebugOutput::callbackImplementationKhrDesktopES32(const Callback callback, const void* userParam) {
+void DebugOutput::callbackImplementationKhrDesktopES32(const Callback callback) {
     /* Replace the callback */
-    const Callback original = Context::current().state().debug.messageCallback.callback;
+    const bool setPreviously = !!Context::current().state().debug.messageCallback.callback;
+
     Context::current().state().debug.messageCallback.callback = callback;
-    Context::current().state().debug.messageCallback.userParam = userParam;
 
     /* Adding callback */
-    if(!original && callback)
+    if(!setPreviously && callback)
         glDebugMessageCallback(callbackWrapper,  &Context::current().state().debug.messageCallback);
 
     /* Deleting callback */
-    else if(original && !callback)
+    else if(setPreviously && !callback)
         glDebugMessageCallback(nullptr, nullptr);
 }
 #endif
 
 #ifdef MAGNUM_TARGET_GLES
-void DebugOutput::callbackImplementationKhrES(const Callback callback, const void* userParam) {
+void DebugOutput::callbackImplementationKhrES(const Callback callback) {
     /* Replace the callback */
-    const Callback original = Context::current().state().debug.messageCallback.callback;
+    const bool setPreviously = !!Context::current().state().debug.messageCallback.callback;
+
     Context::current().state().debug.messageCallback.callback = callback;
-    Context::current().state().debug.messageCallback.userParam = userParam;
 
     /* Adding callback */
-    if(!original && callback)
+    if(!setPreviously && callback)
         glDebugMessageCallbackKHR(callbackWrapper, &Context::current().state().debug.messageCallback);
 
     /* Deleting callback */
-    else if(original && !callback)
+    else if(setPreviously && !callback)
         glDebugMessageCallbackKHR(nullptr, nullptr);
 }
 #endif

src/Magnum/GL/DebugOutput.h

@@ -422,12 +422,12 @@ class MAGNUM_GL_EXPORT DebugOutput {
         static MAGNUM_GL_LOCAL void controlImplementationKhrES(GLenum source, GLenum type, GLenum severity, std::initializer_list<UnsignedInt> ids, bool enabled);
         #endif
 
-        static MAGNUM_GL_LOCAL void callbackImplementationNoOp(Callback, const void*);
+        static MAGNUM_GL_LOCAL void callbackImplementationNoOp(Callback callback);
         #ifndef MAGNUM_TARGET_GLES2
-        static MAGNUM_GL_LOCAL void callbackImplementationKhrDesktopES32(Callback callback, const void* userParam);
+        static MAGNUM_GL_LOCAL void callbackImplementationKhrDesktopES32(Callback callback);
         #endif
         #ifdef MAGNUM_TARGET_GLES
-        static MAGNUM_GL_LOCAL void callbackImplementationKhrES(Callback callback, const void* userParam);
+        static MAGNUM_GL_LOCAL void callbackImplementationKhrES(Callback callback);
         #endif
 };
 

src/Magnum/GL/Implementation/DebugState.h

@@ -42,7 +42,7 @@ struct DebugState {
 
     void(*messageInsertImplementation)(DebugMessage::Source, DebugMessage::Type, UnsignedInt, DebugOutput::Severity, Containers::ArrayView<const char>);
     void(*controlImplementation)(GLenum, GLenum, GLenum, std::initializer_list<UnsignedInt>, bool);
-    void(*callbackImplementation)(DebugOutput::Callback, const void*);
+    void(*callbackImplementation)(DebugOutput::Callback);
     void(*pushGroupImplementation)(DebugGroup::Source, UnsignedInt, Containers::ArrayView<const char>);
     void(*popGroupImplementation)();