Make canvas work without ImDrawList::_FringeScale extension. #25

This change mage _FringeScale extension optional and make canvas
compile without it. SFINAE is used to detect if member variable
is present.

Fringe scale extension to ImDrawList make zoomed geometry look sharp.
It is available here:
https://github.com/thedmd/imgui/tree/feature/draw-list-fringe-scale

NodeEditor/Source/imgui_canvas.cpp

@@ -1,5 +1,62 @@
 # define IMGUI_DEFINE_MATH_OPERATORS
 # include "imgui_canvas.h"
+# include <type_traits>
+
+// https://stackoverflow.com/a/36079786
+# define DECLARE_HAS_MEMBER(__trait_name__, __member_name__)                         \
+                                                                                     \
+    template <typename __boost_has_member_T__>                                       \
+    class __trait_name__                                                             \
+    {                                                                                \
+        using check_type = ::std::remove_const_t<__boost_has_member_T__>;            \
+        struct no_type {char x[2];};                                                 \
+        using  yes_type = char;                                                      \
+                                                                                     \
+        struct  base { void __member_name__() {}};                                   \
+        struct mixin : public base, public check_type {};                            \
+                                                                                     \
+        template <void (base::*)()> struct aux {};                                   \
+                                                                                     \
+        template <typename U> static no_type  test(aux<&U::__member_name__>*);       \
+        template <typename U> static yes_type test(...);                             \
+                                                                                     \
+        public:                                                                      \
+                                                                                     \
+        static constexpr bool value = (sizeof(yes_type) == sizeof(test<mixin>(0)));  \
+    }
+
+namespace ImCanvasDetails {
+
+DECLARE_HAS_MEMBER(HasFringeScale, _FringeScale);
+
+struct FringeScaleRef
+{
+    // Overload is present when ImDrawList does have _FringeScale member variable.
+    template <typename T>
+    static float& Get(typename std::enable_if<HasFringeScale<T>::value, T>::type* drawList)
+    {
+        return drawList->_FringeScale;
+    }
+
+    // Overload is present when ImDrawList does not have _FringeScale member variable.
+    template <typename T>
+    static float& Get(typename std::enable_if<!HasFringeScale<T>::value, T>::type*)
+    {
+        static float placeholder = 1.0f;
+        return placeholder;
+    }
+};
+
+} // namespace ImCanvasDetails
+
+// Returns a reference to _FringeScale extension to ImDrawList
+//
+// If ImDrawList does not have _FringeScale a placeholder is returned.
+static inline float& ImFringeScaleRef(ImDrawList* drawList)
+{
+    using namespace ImCanvasDetails;
+    return FringeScaleRef::Get<ImDrawList>(drawList);
+}
 
 static inline ImVec2 ImSelectPositive(const ImVec2& lhs, const ImVec2& rhs) { return ImVec2(lhs.x > 0.0f ? lhs.x : rhs.x, lhs.y > 0.0f ? lhs.y : rhs.y); }
 
@@ -319,8 +376,9 @@ void ImGuiEx::Canvas::EnterLocalSpace()
 
     m_ViewRect = CalcViewRect(m_View);;
 
-    m_LastFringeScale = m_DrawList->_FringeScale;
-    m_DrawList->_FringeScale *= m_View.InvScale;
+    auto& fringeScale = ImFringeScaleRef(m_DrawList);
+    m_LastFringeScale = fringeScale;
+    fringeScale *= m_View.InvScale;
 }
 
 void ImGuiEx::Canvas::LeaveLocalSpace()
@@ -384,7 +442,8 @@ void ImGuiEx::Canvas::LeaveLocalSpace()
         }
     }
 
-    m_DrawList->_FringeScale = m_LastFringeScale;
+    auto& fringeScale = ImFringeScaleRef(m_DrawList);
+    fringeScale = m_LastFringeScale;
 
     // And pop \o/
     ImGui::PopClipRect();