Fixed build with Visual Studio 2008 or earlier (via PR #203)

imgui.cpp

@@ -400,9 +400,13 @@
 #include "imgui.h"
 #include <ctype.h>      // toupper, isprint
 #include <math.h>       // sqrtf, fabsf, fmodf, powf, cosf, sinf, floorf, ceilf
-#include <stdint.h>     // intptr_t
 #include <stdio.h>      // vsnprintf, sscanf
 #include <new>          // new (ptr)
+#if defined(_MSC_VER) && _MSC_VER <= 1500 // MSVC 2008 or earlier
+#include <stddef.h>     // intptr_t
+#else
+#include <stdint.h>     // intptr_t
+#endif
 
 #ifdef _MSC_VER
 #pragma warning (disable: 4505) // unreferenced local function has been removed (stb stuff)