Sort out header situation for non-SDL builds

...by vendoring them

source/Irrlicht/COpenGLCommon.h

@@ -6,32 +6,18 @@
 
 #ifdef _IRR_COMPILE_WITH_OPENGL_
 
-#if defined(_IRR_COMPILE_WITH_WINDOWS_DEVICE_)
-	#define WIN32_LEAN_AND_MEAN
-	#include <windows.h>
-		#define GL_GLEXT_LEGACY 1
-	#include <GL/gl.h>
-		#include <GL/glext.h>
-	#include <GL/wglext.h>
-#elif defined(_IRR_COMPILE_WITH_OSX_DEVICE_)
-		#define GL_GLEXT_LEGACY 1
-	#include <OpenGL/gl.h>
-		#error glext.h missing on OSX
-#elif defined(_IRR_COMPILE_WITH_SDL_DEVICE_) && !defined(_IRR_COMPILE_WITH_X11_DEVICE_)
-		#define GL_GLEXT_LEGACY 1
+#if defined(_IRR_COMPILE_WITH_SDL_DEVICE_)
 	#include <SDL_video.h>
 	#include <SDL_opengl.h>
-		#ifdef __APPLE__
-		#include <SDL_opengl_glext.h>
-		#else
-		#include <GL/glext.h>
-		#endif
 #else
-		#define GL_GLEXT_LEGACY 1
-		#define GLX_GLXEXT_LEGACY 1
-	#include <GL/gl.h>
+	#include "vendor/gl.h"
+#endif
+
+#if defined(_IRR_COMPILE_WITH_WINDOWS_DEVICE_)
+	#include <GL/wglext.h>
+#elif defined(_IRR_COMPILE_WITH_X11_DEVICE_)
+	#define GLX_GLXEXT_LEGACY 1
 	#include <GL/glx.h>
-	#include <GL/glext.h>
 	#include <GL/glxext.h>
 #endif
 

source/Irrlicht/OpenGL/Common.h

@@ -7,18 +7,11 @@
 
 #include "irrTypes.h"
 // even though we have mt_opengl.h our driver code still uses GL_* constants
-#if defined(_IRR_COMPILE_WITH_WINDOWS_DEVICE_)
-	#include <GL/gl.h>
-	#include <GL/glext.h>
-#elif defined(_IRR_COMPILE_WITH_OSX_DEVICE_)
-	#include <OpenGL/gl.h>
-	// not sure if this works without glext.h?
-#elif defined(_IRR_COMPILE_WITH_SDL_DEVICE_) && !defined(_IRR_COMPILE_WITH_X11_DEVICE_)
+#if defined(_IRR_COMPILE_WITH_SDL_DEVICE_)
 	#include <SDL_video.h>
 	#include <SDL_opengl.h>
 #else
-	#include <GL/gl.h>
-	#include <GL/glext.h>
+	#include "vendor/gl.h"
 #endif
 
 namespace irr