Bug 474886 - Allow rank 0 matrices when setting the font matrix. r=vlad p=jrmuizel

cairo_scaled_font_init() allows rank 0 matrics so cairo_set_font_matrix
should too. This keeps us from accidentally breaking our cairo context
and causing things to draw wrong.

gfx/cairo/cairo/src/cairo-gstate.c

@@ -1271,8 +1271,13 @@ _cairo_gstate_set_font_matrix (cairo_gstate_t	    *gstate,
     if (memcmp (matrix, &gstate->font_matrix, sizeof (cairo_matrix_t)) == 0)
 	return CAIRO_STATUS_SUCCESS;
 
-    if (! _cairo_matrix_is_invertible (matrix))
-	return _cairo_error (CAIRO_STATUS_INVALID_MATRIX);
+    if (! _cairo_matrix_is_invertible (matrix)) {
+	/* rank 0 matrices are ok even though they are not invertible */
+	if (!(matrix->xx == 0. && matrix->xy == 0. &&
+	      matrix->yx == 0. && matrix->yy == 0.)) {
+	    return _cairo_error (CAIRO_STATUS_INVALID_MATRIX);
+	}
+    }
 
     _cairo_gstate_unset_scaled_font (gstate);