Fix a logic error in color glyph compositing

When a color glyph is completely clipped away, we
get the non-zero status 'nothing to do'. In that case,
we must not exit early, since there might still be work
to do for the other color glyphs.

This bug was showing up as color glyphs stopping to render
in GTK+ entries that are scrolled to the right, and also
as color glyphs not rendering inside the selection sometimes.

See https://bugzilla.gnome.org/show_bug.cgi?id=790255 and
https://bugzilla.gnome.org/show_bug.cgi?id=788071.

src/cairo-surface.c

@@ -2742,7 +2742,7 @@ composite_color_glyphs (cairo_surface_t             *surface,
 
                 status = composite_one_color_glyph (surface, op, source, clip,
                                                     &glyphs[gp], scaled_glyph);
-                if (unlikely (status))
+                if (unlikely (status && status != CAIRO_INT_STATUS_NOTHING_TO_DO))
                     goto UNLOCK;
             }
 
@@ -2776,7 +2776,7 @@ composite_color_glyphs (cairo_surface_t             *surface,
 
            status = composite_one_color_glyph (surface, op, source, clip,
                                                &glyphs[glyph_pos], scaled_glyph);
-           if (unlikely (status))
+           if (unlikely (status && status != CAIRO_INT_STATUS_NOTHING_TO_DO))
                goto UNLOCK;
         }
 
@@ -2854,7 +2854,7 @@ _cairo_surface_show_text_glyphs (cairo_surface_t	    *surface,
                                          scaled_font,
                                          clip);
 
-        if (unlikely (status))
+        if (unlikely (status && status != CAIRO_INT_STATUS_NOTHING_TO_DO))
             goto DONE;
 
         if (num_glyphs == 0)