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

tests/test_commands.py

@@ -25,6 +25,7 @@ class FakeTerminalView:
         self._id = FakeTerminalView._next_id
         FakeTerminalView._next_id += 1
         self._settings = sublime.Settings()
+        self._settings.set("vterm_view", True)
 
     def id(self):
         return self._id
@@ -36,7 +37,24 @@ class FakeTerminalView:
         return True
 
     def style(self):
-        return {}
+        # a terminal view reports its own theme's colours, not the editor's
+        return {"foreground": "#f2f2f2", "background": "#000000"}
+
+
+class FakeEditorView:
+    """An ordinary file view, the one the "No theme" palette is sampled from."""
+
+    def settings(self):
+        return sublime.Settings()
+
+    def is_valid(self):
+        return True
+
+    def style(self):
+        return {"foreground": "#111111", "background": "#eeeeee"}
+
+
+editor = FakeEditorView()
 
 
 class FakeWindow:
@@ -46,6 +64,9 @@ class FakeWindow:
     def active_view(self):
         return self._view
 
+    def views(self):
+        return [self._view, editor]
+
     def show_quick_panel(self, items, on_done, selected_index=-1, on_highlight=None):
         panel.update(items=items, on_done=on_done,
                      selected_index=selected_index, on_highlight=on_highlight)
@@ -105,6 +126,12 @@ command = open_picker(fake_terminal("slate"))
 panel["on_done"](0)
 assert settings.get("theme") is None, settings.get("theme")
 
+# ... and samples the editor's colours, not the themed terminal's own: that
+# would retint every "no theme" terminal with the theme being left
+system = get_scheme_manager(None)
+assert (system.foreground, system.background) == ("#111111", "#eeeeee"), \
+    (system.foreground, system.background)
+
 print("theme picker tests passed ({} themes offered)".format(len(THEMES)))