Reorganized context handling to be more explicit,

- YOU NOW NEED TO CALL ImGui::CreateContext() AT THE BEGINNING OF YOUR APP, AND CALL ImGui::DestroyContext() AT THE END.
- removed Shutdown() function, as DestroyContext() serve this purpose.
- you may pass a ImFontAtlas* pointer to CreateContext() to share a font atlas between contexts. Otherwhise CreateContext() will create its own font atlas instance.
- removed allocator parameters from CreateContext(), they are now setup with SetAllocatorFunctions(), and shared by all contexts.
- removed the default global context and font atlas instance, which were confusing for users of DLL reloading and users of multiple contexts
(#1565, #586, #992, #1007, #1558)

imgui.h

@@ -127,6 +127,14 @@ struct ImVec4
 // In a namespace so that user can add extra functions in a separate file (e.g. Value() helpers for your vector or common types)
 namespace ImGui
 {
+    // Context creation and access, if you want to use multiple context, share context between modules (e.g. DLL). 
+    // All contexts share a same ImFontAtlas by default. If you want different font atlas, you can new() them and overwrite the GetIO().Fonts variable of an ImGui context.
+    // All those functions are not reliant on the current context.
+    IMGUI_API ImGuiContext* CreateContext(ImFontAtlas* shared_font_atlas = NULL);
+    IMGUI_API void          DestroyContext(ImGuiContext* ctx = NULL);   // NULL = Destroy current context
+    IMGUI_API ImGuiContext* GetCurrentContext();
+    IMGUI_API void          SetCurrentContext(ImGuiContext* ctx);
+
     // Main
     IMGUI_API ImGuiIO&      GetIO();
     IMGUI_API ImGuiStyle&   GetStyle();
@@ -134,7 +142,6 @@ namespace ImGui
     IMGUI_API void          NewFrame();                                 // start a new ImGui frame, you can submit any command from this point until Render()/EndFrame().
     IMGUI_API void          Render();                                   // ends the ImGui frame, finalize the draw data, then call your io.RenderDrawListsFn() function if set.
     IMGUI_API void          EndFrame();                                 // ends the ImGui frame. automatically called by Render(), so most likely don't need to ever call that yourself directly. If you don't need to render you may call EndFrame() but you'll have wasted CPU already. If you don't need to render, better to not create any imgui windows instead!
-    IMGUI_API void          Shutdown();
 
     // Demo, Debug, Informations
     IMGUI_API void          ShowDemoWindow(bool* p_open = NULL);        // create demo/test window (previously called ShowTestWindow). demonstrate most ImGui features. call this to learn about the library! try to make it always available in your application!
@@ -143,6 +150,7 @@ namespace ImGui
     IMGUI_API bool          ShowStyleSelector(const char* label);
     IMGUI_API void          ShowFontSelector(const char* label);
     IMGUI_API void          ShowUserGuide();                            // add basic help/info block (not a window): how to manipulate ImGui as a end-user (mouse/keyboard controls).
+    IMGUI_API const char*   GetVersion();
 
     // Window
     IMGUI_API bool          Begin(const char* name, bool* p_open = NULL, ImGuiWindowFlags flags = 0);   // push window to the stack and start appending to it. see .cpp for details. return false when window is collapsed (so you can early out in your code) but you always need to call End() regardless. 'bool* p_open' creates a widget on the upper-right to close the window (which sets your bool to false).
@@ -510,15 +518,6 @@ namespace ImGui
     IMGUI_API const char*   GetClipboardText();
     IMGUI_API void          SetClipboardText(const char* text);
 
-    // Context creation and access, if you want to use multiple context, share context between modules (e.g. DLL). There is a default context created and active by default.
-    // All contexts share a same ImFontAtlas by default. If you want different font atlas, you can new() them and overwrite the GetIO().Fonts variable of an ImGui context.
-    // All those functions are not reliant on the current context.
-    IMGUI_API const char*   GetVersion();
-    IMGUI_API ImGuiContext* CreateContext();
-    IMGUI_API void          DestroyContext(ImGuiContext* ctx);
-    IMGUI_API ImGuiContext* GetCurrentContext();
-    IMGUI_API void          SetCurrentContext(ImGuiContext* ctx);
-
 } // namespace ImGui
 
 // Flags for ImGui::Begin()