opengl: use OpenGL ES 2 on iOS and fix a compilation warning

modules/video_output/opengl.h

@@ -31,21 +31,22 @@
 
 /* Change USE_OPENGL_ES value to set the OpenGL ES version (1, 2) you want to use
  * A value of 0 will activate normal OpenGL */
-#ifndef USE_OPENGL_ES
-# define USE_OPENGL_ES 0
-#endif
 #ifdef __APPLE__
 # include <TargetConditionals.h>
 # if !TARGET_OS_IPHONE
+#  define USE_OPENGL_ES 0
 #  define MACOS_OPENGL
 #  include <OpenGL/gl.h>
-# else /* Force ESv1 on iOS */
-#  define USE_OPENGL_ES 1
+# else /* Force ESv2 on iOS */
+#  define USE_OPENGL_ES 2
 #  include <OpenGLES/ES1/gl.h>
 #  include <OpenGLES/ES2/gl.h>
 #  include <OpenGLES/ES2/glext.h>
 # endif
 #else /* !defined (__APPLE__) */
+# ifndef USE_OPENGL_ES
+#  define USE_OPENGL_ES 0
+# endif
 # if USE_OPENGL_ES == 2
 #  include <GLES2/gl2.h>
 # elif USE_OPENGL_ES == 1