[mutter] cogl: Fix some misleading variable names

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 68166f33d9f8221e829ad87cdd214f17c00ecd6a
Author: Adam Jackson <ajax redhat com>
Date:   Thu Mar 7 17:49:59 2019 -0500

    cogl: Fix some misleading variable names
    
    This was clearly blindly copypasta'd from the (now deleted) 3D texture
    code.
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/487

 cogl/cogl/cogl-glsl-shader.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/cogl/cogl/cogl-glsl-shader.c b/cogl/cogl/cogl-glsl-shader.c
index 6e1deb66c..fda923c34 100644
--- a/cogl/cogl/cogl-glsl-shader.c
+++ b/cogl/cogl/cogl-glsl-shader.c
@@ -102,10 +102,10 @@ _cogl_glsl_shader_set_source_with_boilerplate (CoglContext *ctx,
 
   if (cogl_has_feature (ctx, COGL_FEATURE_ID_TEXTURE_EGL_IMAGE_EXTERNAL))
     {
-      static const char texture_3d_extension[] =
+      static const char image_external_extension[] =
         "#extension GL_OES_EGL_image_external : require\n";
-      strings[count] = texture_3d_extension;
-      lengths[count++] = sizeof (texture_3d_extension) - 1;
+      strings[count] = image_external_extension;
+      lengths[count++] = sizeof (image_external_extension) - 1;
     }
 
   if (shader_gl_type == GL_VERTEX_SHADER)