cogl-pango-glyph-cache: Fix the flags passed to cogl_atlas_new

In 047227fb cogl_atlas_new was changed so that it can take a flags
parameter to specify whether to clear the new atlases and whether to
copy images to the new atlas after reorganisation. This was done so
that the atlas code could be shared with the glyph cache. At some
point during the development of this patch the flag was just a single
boolean instead and this is accidentally how it is used from the glyph
cache. The glyph cache therefore passes 'TRUE' as the set of flags
which means it will only get the 'clear' flag and not the
'disable-migration' flag. When the glyph cache gets full it will
therefore try to copy the texture to the new atlas as well as
redrawing them with cairo. This causes problems because the glyph
cache needs to work in situations where there is no FBO support.

pango/cogl-pango-glyph-cache.c

@@ -225,7 +225,8 @@ cogl_pango_glyph_cache_lookup (CoglPangoGlyphCache *cache,
       if (atlas == NULL)
         {
           atlas = _cogl_atlas_new (COGL_PIXEL_FORMAT_A_8,
-                                   TRUE,
+                                   COGL_ATLAS_CLEAR_TEXTURE |
+                                   COGL_ATLAS_DISABLE_MIGRATION,
                                    cogl_pango_glyph_cache_update_position_cb);
           COGL_NOTE (ATLAS, "Created new atlas for glyphs: %p", atlas);
           /* If we still can't reserve space then something has gone