Fix text loss after zero-width chars, grey 248 quantization, scrollback cap, no-theme sampling, shipped terminal default

- draw(): a zero-width non-combining char (VS16, ZWJ) or a stray control
  byte dropped the rest of the run; skip the char instead
- quantize_256: grey 248 mapped to slot 256 and aborted the SGR
- trim_history: the cap now counts rows above the screen, so a value
  below the screen height (0: no scrollback) no longer cuts the screen
- "No theme" sampled the focused terminal's own colours; sample a
  non-terminal view instead
- vterm.sublime-settings: Apple Terminal is the default again, kitty
  commented out, as the README says

vterm.sublime-settings

@@ -33,12 +33,12 @@
 
     // External terminal for "vterm: Open Terminal in Project Folder".
     // %CWD% in parameters is replaced with the project folder.
-    // "terminal": "/usr/bin/open",
-    // "parameters": ["-a", "Terminal", "%CWD%"],
+    "terminal": "/usr/bin/open",
+    "parameters": ["-a", "Terminal", "%CWD%"],
 
     // kitty instead of Apple Terminal:
-    "terminal": "/opt/homebrew/bin/kitty",
-    "parameters": ["-d", "%CWD%"],
+    // "terminal": "/opt/homebrew/bin/kitty",
+    // "parameters": ["-d", "%CWD%"],
 
     "debug": false
 }