ChafaSixelCanvas: Reintroduce sixel transparency

We can clear the background in animation frames by setting the alpha
threshold now, like we do for Kitty.

Ref: #211 (GitHub).
Ref: #147 (GitHub).

chafa/internal/chafa-sixel-canvas.c

@@ -499,7 +499,7 @@ chafa_sixel_canvas_build_ansi (ChafaSixelCanvas *sixel_canvas, ChafaTermInfo *te
 
     chafa_passthrough_encoder_begin (&ptenc, passthrough, term_info, str);
 
-    *chafa_term_info_emit_begin_sixels (term_info, buf, 0, 0, 0) = '\0';
+    *chafa_term_info_emit_begin_sixels (term_info, buf, 0, 1, 0) = '\0';
     chafa_passthrough_encoder_append (&ptenc, buf);
 
     g_snprintf (buf,