32_gl_interop/main.cpp: fix error and warnings

On linux  g++ headers more "clean" and doesn't implicitly include <memory>. Include <memory> header to avoid compilation error
GLuint = NUL provides warning at compilation time "warning: converting to non-pointer type". GLuint should be = 0

Author: feniksa

tutorials/32_gl_interop/main.cpp

@@ -45,6 +45,7 @@
 #include <cassert>
 #include <iostream>
 #include <thread>
+#include <memory>
 
 const unsigned int WINDOW_WIDTH = 640;
 const unsigned int WINDOW_HEIGHT = 480;
@@ -84,9 +85,9 @@ class GuiRenderImpl
 };
 
 
-GLuint              g_vertex_buffer_id = NULL;
-GLuint              g_index_buffer_id = NULL;
-GLuint              g_texture = NULL;
+GLuint              g_vertex_buffer_id = 0;
+GLuint              g_index_buffer_id = 0;
+GLuint              g_texture = 0;
 rpr_framebuffer		g_frame_buffer = NULL;
 rpr_context         g_context = NULL;
 rpr_material_system g_matsys = NULL;