Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

src/textfile_gui.cpp

@@ -366,8 +366,21 @@ static void Xunzip(byte **bufp, size_t *sizep)
 	if (StrStartsWith(sv_buf, u8"\ufeff")) sv_buf.remove_prefix(3);
 
 	/* Replace any invalid characters with a question-mark. This copies the buf in the process. */
-	this->text = StrMakeValid(sv_buf, SVS_REPLACE_WITH_QUESTION_MARK | SVS_ALLOW_NEWLINE | SVS_REPLACE_TAB_CR_NL_WITH_SPACE);
+	this->LoadText(sv_buf);
 	free(buf);
+}
+
+/**
+ * Load a text into the textfile viewer.
+ *
+ * This will split the text into newlines and stores it for fast drawing.
+ *
+ * @param buf The text to load.
+ */
+void TextfileWindow::LoadText(std::string_view buf)
+{
+	this->text = StrMakeValid(buf, SVS_REPLACE_WITH_QUESTION_MARK | SVS_ALLOW_NEWLINE | SVS_REPLACE_TAB_CR_NL_WITH_SPACE);
+	this->lines.clear();
 
 	/* Split the string on newlines. */
 	std::string_view p(this->text);
@@ -406,7 +419,7 @@ std::optional<std::string> GetTextfile(TextfileType type, Subdirectory dir, cons
 		"changelog",
 		"license",
 	};
-	static_assert(lengthof(prefixes) == TFT_END);
+	static_assert(lengthof(prefixes) == TFT_CONTENT_END);
 
 	std::string_view prefix = prefixes[type];