sixel: implement P2=1 - transparent pixels

When P2=1, empty pixels are transparent.

This patch also changes the behavior of P2=0|2, from setting empty
pixels to the default background color, to instead use the *current*
background color.

To implement this, a couple of changes are needed:

* Sixel pixels always use alpha=1.0, except for *empty* cells when
  P2=1 (i.e. transparent pixels).
* The renderer draws sixels with the OVER operator, instead of the SRC
  operator.
* The renderer *must* now render the cells beneath the sixel. As an
  optimization, this is only done for sixels where P2=1. I.e. for
  fully opaque sixels, there’s no need to render the cells beneath.

The sixel renderer isn’t yet hooked into the multi-threaded
renderer. This means *rows* (not just the cells) beneath
maybe-transparent sixels are rendered single-threaded.

Closes #391.

sixel.h

@@ -8,7 +8,7 @@
 
 void sixel_fini(struct terminal *term);
 
-void sixel_init(struct terminal *term);
+void sixel_init(struct terminal *term, int p1, int p2, int p3);
 void sixel_put(struct terminal *term, uint8_t c);
 void sixel_unhook(struct terminal *term);