Delete duplicate GL headers

Nothing was pointing at BBGE/glext.

Only the VS project was pointing at ExternalLibs/GL. I have pointed it
at the headers under BBGE/GL instead.

I have also adjusted the includes to use the GL prefix as is standard
practise. These will still work as BBGE is in the include path. This
allows users to build against their system GL headers simply by
deleting the BBGE/GL directory.

ExternalLibs/GL/gles.h

@@ -1,14 +0,0 @@
-/* OpenGL ES-specific. --ryan. */
-/* I should probably put an official GLES header in here instead. Oh well. */
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-void glOrthof(GLfloat left, GLfloat right, GLfloat bottom, GLfloat top, GLfloat zNear, GLfloat zFar);
-void glClearDepthf(GLclampf depth);
-
-#ifdef __cplusplus
-}
-#endif
-

ExternalLibs/glfont2/glfont2.cpp

@@ -19,7 +19,7 @@ using namespace std;
 #include <windows.h>
 #undef GetCharWidth
 #endif
-#include "gl.h"
+#include <GL/gl.h>
 
 //glFont header
 #include "glfont2.h"

ExternalLibs/glpng/glpng.c

@@ -31,11 +31,9 @@
 //#define GL_GLEXT_PROTOTYPES
 
 //#include <GL/glpng.h>
-//#include <GL/gl.h>
-//#include <GL/glext.h>
 #include "../glpng.h"
-#include "gl.h"
-#include "glext.h"
+#include <GL/gl.h>
+#include <GL/glext.h>
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>