Roblox backend: one layer (Frame + tiles) per draw list so moving windows costs no redraw; gradient rect fast path; t_drag/t_composite checks

tools/test/compare_composite.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+# Compare two t_composite.luau outputs: compare_composite.py a.txt b.txt
+import sys
+def load(p):
+    rows = {}
+    for l in open(p):
+        if l.startswith("ROW"):
+            _, y, h = l.split(); rows[int(y)] = bytes.fromhex(h)
+    return rows
+a, b = load(sys.argv[1]), load(sys.argv[2])
+assert a and len(a) == len(b), "missing/uneven output"
+diff = big = 0; worst = 0
+for y in a:
+    for x, (p, q) in enumerate(zip(a[y], b[y])):
+        d = abs(p - q)
+        if d: diff += 1
+        if d > 2: big += 1
+        worst = max(worst, d)
+print("channels differing: %d, by more than 2: %d, max diff: %d" % (diff, big, worst))
+sys.exit(1 if big else 0)