Don't instanitate/declare structs as classes.

Clang warned about that.

src/AbstractTexture.cpp

@@ -458,9 +458,9 @@ namespace Implementation {
         return nullptr;
     }
 
-    template class ImageHelper<BufferImage1D>;
+    template struct ImageHelper<BufferImage1D>;
-    template class ImageHelper<BufferImage2D>;
+    template struct ImageHelper<BufferImage2D>;
-    template class ImageHelper<BufferImage3D>;
+    template struct ImageHelper<BufferImage3D>;
 }
 #endif
 

src/Context.h

@@ -30,7 +30,7 @@ namespace Magnum {
 
 #ifndef DOXYGEN_GENERATING_OUTPUT
 namespace Implementation {
-    class State;
+    struct State;
 }
 #endif
 

src/Magnum.h

@@ -53,7 +53,7 @@ namespace Magnum {
 namespace Math {
     template<class T> constexpr T deg(T value);
     template<class T> constexpr T rad(T value);
-    template<class T> class Constants;
+    template<class T> struct Constants;
 }
 
 /* Bring debugging facility from Corrade::Utility namespace */