Made some constructors constexpr.

src/Implementation/FramebufferState.h

@@ -30,7 +30,7 @@
 namespace Magnum { namespace Implementation {
 
 struct FramebufferState {
-    inline FramebufferState(): readBinding(0), drawBinding(0), renderbufferBinding(0) {}
+    inline constexpr FramebufferState(): readBinding(0), drawBinding(0), renderbufferBinding(0) {}
 
     GLuint readBinding, drawBinding, renderbufferBinding;
     Rectanglei viewport;

src/Implementation/MeshState.h

@@ -29,7 +29,7 @@
 namespace Magnum { namespace Implementation {
 
 struct MeshState {
-    inline MeshState(): currentVAO(0) {}
+    inline constexpr MeshState(): currentVAO(0) {}
 
     GLuint currentVAO;
 };