opengl: fix win32 build

it is not clear how glew(Active)ClientTexture should be acceeded,
do not use the broken glew 'shortcut' for these functions

modules/video_output/opengl.h

@@ -55,6 +55,10 @@
 # else
 #  ifdef WIN32
 #   include <GL/glew.h>
+#   undef glClientActiveTexture
+#   undef glActiveTexture
+    PFNGLACTIVETEXTUREPROC glActiveTexture;
+    PFNGLCLIENTACTIVETEXTUREPROC glClientActiveTexture;
 #  endif
 #  include <GL/gl.h>
 # endif