GDI+ nk_gdip_fill_rect background fix

1. Main background area was filled by 2 big overlapping rectangles. It works good on solid but not on semi-transparent themes. Just replaced 2 big rectangles with 1 big and 2 small.
2. There is no need in [nk_gdip_fill_rect filled pies fix](https://github.com/vurtun/nuklear/commit/7ab7327fa492298d5fa85a40340892905f3995fc) anymore. It disturbs semi-transparent themes too.

demo/gdip/nuklear_gdip.h

@@ -458,8 +458,9 @@ nk_gdip_fill_rect(short x, short y, unsigned short w,
         GdipFillRectangleI(gdip.memory, gdip.brush, x, y, w, h);
     } else {
         INT d = 2 * r;
-        GdipFillRectangleI(gdip.memory, gdip.brush, x + r - 1, y, w - d + 2, h);
-        GdipFillRectangleI(gdip.memory, gdip.brush, x, y + r - 1, w, h - d + 2);
+        GdipFillRectangleI(gdip.memory, gdip.brush, x + r, y, w - d, h);
+        GdipFillRectangleI(gdip.memory, gdip.brush, x, y + r, r, h - d);
+        GdipFillRectangleI(gdip.memory, gdip.brush, x + w - r, y + r, r, h - d);
         GdipFillPieI(gdip.memory, gdip.brush, x, y, d, d, 180, 90);
         GdipFillPieI(gdip.memory, gdip.brush, x + w - d, y, d, d, 270, 90);
         GdipFillPieI(gdip.memory, gdip.brush, x + w - d, y + h - d, d, d, 0, 90);