Fixed non displayed windows.

WARNING: Drawing is not done correctly

demo/gdi_native_nuklear/main.c

@@ -16,23 +16,39 @@ processorArchitecture='*' publicKeyToken='6595b64144ccf1df' language='*'\"")
 #define NKGDI_IMPLEMENT_WINDOW
 #include "window.h"
 
+int drawCallback(struct nk_context* ctx)
+{
+    nk_label(ctx, "Label", NK_TEXT_ALIGN_CENTERED);
+    nk_button_label(ctx, "Test 1234");
+    return 1;
+}
+
 INT WINAPI wWinMain(HINSTANCE _In_ hInstance, HINSTANCE _In_opt_ hPrevInstance, PWSTR _In_ cmdArgs, INT _In_ cmdShow)
 {
+    nkgdi_window_init();
     struct nkgdi_window w1, w2;
+    memset(&w1, 0x0, sizeof(struct nkgdi_window));
+    memset(&w2, 0x0, sizeof(struct nkgdi_window));
 
     w1.allow_sizing = 0;
     w1.allow_maximize = 0;
     w1.allow_move = 0;
     w1.has_titlebar = 0;
+    w1.cb_on_draw = &drawCallback;
     nkgdi_window_create(&w1, 500, 500, "F1", 10, 10);
 
     w2.allow_sizing = 1;
     w2.allow_maximize = 1;
     w2.allow_move = 1;
     w2.has_titlebar = 1;
+    w2.cb_on_draw = &drawCallback;
     nkgdi_window_create(&w2, 500, 500, "F2", 520, 10);
     
     while (nkgdi_window_update(&w1) && nkgdi_window_update(&w2)) Sleep(20);
 
+    nkgdi_window_destroy(&w1);
+    nkgdi_window_destroy(&w2);
+
+    nkgdi_window_shutdown();
     return 0;
 }

demo/gdi_native_nuklear/window.h

@@ -130,8 +130,8 @@ void nkgdi_window_create(struct nkgdi_window* wnd, unsigned int width, unsigned
     wnd->_internal.is_maximized = 0;
     wnd->_internal.drag_offset.x = 0;
     wnd->_internal.drag_offset.y = 0;
-    wnd->_internal.width = 0;
-    wnd->_internal.height = 0;
+    wnd->_internal.width = width;
+    wnd->_internal.height = height;
 }
 
 void nkgdi_window_destroy(struct nkgdi_window* wnd)
@@ -193,7 +193,7 @@ int nkgdi_window_update(struct nkgdi_window* wnd)
             nk_window_set_bounds(wnd->_internal.nk_ctx, title, nk_rect(0, 0, wnd->_internal.width, wnd->_internal.height));
         if (nk_begin(wnd->_internal.nk_ctx, title, nk_rect(0, 0, wnd->_internal.width, wnd->_internal.height), window_flags))
         {
-            if(!wnd->cb_on_draw(wnd->_internal.nk_ctx))
+            if(wnd->cb_on_draw && !wnd->cb_on_draw(wnd->_internal.nk_ctx))
                 wnd->_internal.is_open = 0;
 
             // Update window size