kitty: use supplied alpha value directly

In Sixel, we map the alpha value to either 0 or 255, as
it has no alpha concept. Kitty can freely reproduce eight
bits of alpha, so go ahead and use them. Improves image
quality of translucent bitmaps in Kitty at the expense of
some behavioral divergence depending on bitmap backend.

src/lib/kitty.c

@@ -59,7 +59,12 @@ base64_rgba3(const uint32_t* pixels, size_t pcount, char* b64, bool wipe[static
   unsigned r = ncpixel_r(pixel);
   unsigned g = ncpixel_g(pixel);
   unsigned b = ncpixel_b(pixel);
-  unsigned a = wipe[0] ? 0 : rgba_trans_p(pixel, transcolor) ? 0 : 255;
+  // we go ahead and take advantage of kitty's ability to reproduce 8-bit
+  // alphas by copying it in directly, rather than mapping to {0, 255}.
+  unsigned a = ncpixel_a(pixel);
+  if(wipe[0] || rgba_trans_p(pixel, transcolor)){
+    a = 0;
+  }
   b64[0] = b64subs[(r & 0xfc) >> 2];
   b64[1] = b64subs[(r & 0x3 << 4) | ((g & 0xf0) >> 4)];
   b64[2] = b64subs[((g & 0xf) << 2) | ((b & 0xc0) >> 6)];