MSVC 2015 compatibility: in-class init of constexpr is not implemented.

Breaks backward compatibility in one place, but I think that feature
wasn't widely used anyway so I'm not fixing that.

src/Magnum/Implementation/FramebufferState.cpp

@@ -33,7 +33,11 @@
 
 namespace Magnum { namespace Implementation {
 
+#ifndef CORRADE_MSVC2015_COMPATIBILITY
 constexpr const Range2Di FramebufferState::DisengagedViewport;
+#else
+const Range2Di FramebufferState::DisengagedViewport{{}, {-1, -1}};
+#endif
 
 FramebufferState::FramebufferState(Context& context, std::vector<std::string>& extensions): readBinding{0}, drawBinding{0}, renderbufferBinding{0}, maxDrawBuffers{0}, maxColorAttachments{0}, maxRenderbufferSize{0},
     #if !(defined(MAGNUM_TARGET_WEBGL) && defined(MAGNUM_TARGET_GLES2))

src/Magnum/Implementation/FramebufferState.h

@@ -33,7 +33,13 @@
 namespace Magnum { namespace Implementation {
 
 struct FramebufferState {
+    #ifndef CORRADE_MSVC2015_COMPATIBILITY
+    /* MSVC complains that in-class initialization is not yet implemented */
     constexpr static const Range2Di DisengagedViewport{{}, {-1, -1}};
+    #else
+    static const Range2Di DisengagedViewport;
+    #endif
+
 
     explicit FramebufferState(Context& context, std::vector<std::string>& extensions);
 

src/Magnum/Math/Matrix.h

@@ -54,6 +54,9 @@ template<std::size_t size, class T> class Matrix: public RectangularMatrix<size,
         };
 
         #ifdef MAGNUM_BUILD_DEPRECATED
+        /* Sorry, MSVC complains that in-class initialization is not yet
+           implemented and it is not worth fixing */
+        #ifndef CORRADE_MSVC2015_COMPATIBILITY
         /**
          * @brief Pass to constructor to create zero-filled matrix
          * @deprecated Use @ref ZeroInitT and @ref ZeroInit instead.
@@ -76,6 +79,7 @@ template<std::size_t size, class T> class Matrix: public RectangularMatrix<size,
         CORRADE_DEPRECATED("use Math::IdentityInit instead") constexpr static IdentityInitT Identity{};
         #endif
         #endif
+        #endif
 
         /**
          * @brief Default constructor