Shaders: wait, how did the linker work for the whole decade?!

No issue on GCC, only Clang and MSVC started failing once I renamed the
shaders. These four are the only that don't have the export macro
attached AND YET it somehow worked without any problem until now.

src/Magnum/Shaders/DistanceFieldVectorGL.cpp

@@ -145,8 +145,8 @@ template<UnsignedInt dimensions> DistanceFieldVectorGL<dimensions>& DistanceFiel
     return *this;
 }
 
-template class DistanceFieldVectorGL<2>;
-template class DistanceFieldVectorGL<3>;
+template class MAGNUM_SHADERS_EXPORT DistanceFieldVectorGL<2>;
+template class MAGNUM_SHADERS_EXPORT DistanceFieldVectorGL<3>;
 
 namespace Implementation {
 

src/Magnum/Shaders/FlatGL.cpp

@@ -191,8 +191,8 @@ template<UnsignedInt dimensions> FlatGL<dimensions>& FlatGL<dimensions>::setObje
 }
 #endif
 
-template class FlatGL<2>;
-template class FlatGL<3>;
+template class MAGNUM_SHADERS_EXPORT FlatGL<2>;
+template class MAGNUM_SHADERS_EXPORT FlatGL<3>;
 
 namespace Implementation {
 

src/Magnum/Shaders/VectorGL.cpp

@@ -130,8 +130,8 @@ template<UnsignedInt dimensions> VectorGL<dimensions>& VectorGL<dimensions>::set
     return *this;
 }
 
-template class VectorGL<2>;
-template class VectorGL<3>;
+template class MAGNUM_SHADERS_EXPORT VectorGL<2>;
+template class MAGNUM_SHADERS_EXPORT VectorGL<3>;
 
 namespace Implementation {
 

src/Magnum/Shaders/VertexColorGL.cpp

@@ -97,7 +97,7 @@ template<UnsignedInt dimensions> VertexColorGL<dimensions>& VertexColorGL<dimens
     return *this;
 }
 
-template class VertexColorGL<2>;
-template class VertexColorGL<3>;
+template class MAGNUM_SHADERS_EXPORT VertexColorGL<2>;
+template class MAGNUM_SHADERS_EXPORT VertexColorGL<3>;
 
 }}