Merge image_merge into 4902-image-support

Replace SIXEL retention and the per-client cache with the global indexed-image cache from image_merge. Adapt its backend callbacks for Kitty and test palette quantization through both input protocols.

regress/image-sixel-palette.sh

@@ -18,28 +18,23 @@ palette = [(r, g, b) for r in levels for g in levels for b in levels]
 expected = [tuple((c * 255 + 50) // 100 for c in p) for p in palette]
 with image_test(sys.argv[1]) as test:
     client = test.attach()
-    test.emit(sixel(32, 16, palette, list(range(512))))
-    images = client.images()
-    assert len(images) == 1 and images[0][:2] == (32, 16)
-    assert images[0][2] == expected, 'original SIXEL palette changed'
-    test.redraw()
-    assert client.images() == images, 'retained palette changed on redraw'
-
-    # Kitty input exercises RGBA quantization rather than SIXEL retention.
     rgba = bytes(c for pixel in expected for c in (*pixel, 255))
-    test.emit(b'\033[2J\033[H\033_Ga=T,q=2,f=32,s=32,v=16;' +
+    kitty = (b'\033_Ga=T,q=2,f=32,s=32,v=16;' +
         base64.b64encode(rgba) + b'\033\\')
-    images = client.images()
-    assert len(images) == 1 and images[0][:2] == (32, 16)
-    actual = images[0][2]
-    assert all(c is not None for c in actual), 'dithering introduced holes'
-    assert 128 <= len(set(actual)) <= 256, len(set(actual))
-    errors = [abs(a - e) for p, q in zip(actual, expected) for a, e in zip(p, q)]
-    assert max(errors) <= 80, max(errors)
-    assert sum(errors) / len(errors) <= 16, sum(errors) / len(errors)
-    assert max(actual[0]) <= 30 and min(actual[-1]) >= 225, (
-        'black/white extremes lost', actual[0], actual[-1])
-    for _ in range(3):
-        test.redraw()
-        assert client.images() == images, 'palette or dithering changed on redraw'
+    # Both input protocols use the shared RGBA model and quantization.
+    for wire in (sixel(32, 16, palette, list(range(512))), kitty):
+        test.emit(b'\033[2J\033[H' + wire)
+        images = client.images()
+        assert len(images) == 1 and images[0][:2] == (32, 16)
+        actual = images[0][2]
+        assert all(c is not None for c in actual), 'dithering introduced holes'
+        assert 128 <= len(set(actual)) <= 256, len(set(actual))
+        errors = [abs(a - e) for p, q in zip(actual, expected) for a, e in zip(p, q)]
+        assert max(errors) <= 80, max(errors)
+        assert sum(errors) / len(errors) <= 16, sum(errors) / len(errors)
+        assert max(actual[0]) <= 30 and min(actual[-1]) >= 225, (
+            'black/white extremes lost', actual[0], actual[-1])
+        for _ in range(3):
+            test.redraw()
+            assert client.images() == images, 'palette or dithering changed on redraw'
 PY