Fix frame-ordering backspacing empty line bug in script editor

There is a long-standing bug with the script editor where if you delete
the last character of a line, it IMMEDIATELY deletes the line you're on,
and then moves your cursor back to the previous line. This is annoying,
to say the least.

The reason for this is that, in the sequence of events that happens in
one frame (known as frame ordering), the code that backspaces one
character from the line when you press Backspace is ran BEFORE the code
to remove an empty line if you backspace it is ran. The former is
located in key.Poll(), and the latter is located in editorinput().

Thus, when you press Backspace, the game first runs key.Poll(), sees
that you've pressed Backspace, and dutifully removes the last character
from a line. The line is now empty. Then, when the game gets around to
the "Are you pressing Backspace on an empty line?" check in
editorinput(), it thinks that you're pressing Backspace on an empty
line, and then does the usual line-removing stuff.

And actually, when it does the check in editorinput(), it ACTUALLY asks
"Are you pressing Backspace on THIS frame and was the line empty LAST
frame?" because it's checking against its own copy of the input buffer,
before copying the input buffer to its own local copy. So the problem
only happens if you press and hold Backspace for more than 1 frame.
It's a small consolation prize for this annoyance, getting to
tap-tap-tap Backspace in the hopes that you only press it for 1 frame,
while in the middle of something more important to do like, oh I don't
know, writing a script.

So there are two potential solutions here:

 (1) Just change the frame ordering around.

     This is risky to say the least, because I'm not sure what behavior
     depends on exactly which frame order. It's not like it's key.Poll()
     and then IMMEDIATELY afterwards editorinput() is run, it's more
     like key.Poll(), some things that obviously depend on key.Poll()
     running before them, and THEN editorinput(). Also, editorinput() is
     only one possible thing that could be ran afterwards, on the next
     frame we could be running something else entirely instead.

 (2) Add a kludge variable to signal when the line is ALREADY empty so
     the game doesn't re-check the already-empty line and conclude that
     you're already immediately backspacing an empty line.

I went with (2) for this commit, and I've added the kludge variable
key.linealreadyemptykludge.

However, that by itself isn't enough to fix it. It only adds about a
frame or so of delay before the game goes right back to saying "Oh,
you're ALREADY somehow pressing backspace again? I'll just delete this
line real quick" and the behavior is basically the same as before,
except now you have to hit Backspace for TWO frames or less instead of
one in order to not have it happen.

What we need is to have a delay set as well, when the game deletes the
last line of a char. So I set ed.keydelay to 6 as well if editorinput()
sses that key.linealreadyemptykludge is on.

desktop_version/src/KeyPoll.cpp

@@ -50,6 +50,8 @@ KeyPoll::KeyPoll()
 			useFullscreenSpaces = (strcmp(hint, "1") == 0);
 		}
 	}
+
+	linealreadyemptykludge = false;
 }
 
 void KeyPoll::enabletextentry()
@@ -94,7 +96,12 @@ void KeyPoll::Poll()
 			{
 				if (evt.key.keysym.sym == SDLK_BACKSPACE)
 				{
+					bool kbemptybefore = keybuffer.empty();
 					keybuffer = keybuffer.substr(0, keybuffer.length() - 1);
+					if (!kbemptybefore && keybuffer.empty())
+					{
+						linealreadyemptykludge = true;
+					}
 				}
 				else if (	evt.key.keysym.sym == SDLK_v &&
 						keymap[SDLK_LCTRL]	)

desktop_version/src/KeyPoll.h

@@ -71,6 +71,8 @@ public:
 	bool pressedbackspace;
 	std::string keybuffer;
 
+	bool linealreadyemptykludge;
+
 private:
 	std::map<SDL_JoystickID, SDL_GameController*> controllers;
 	std::map<SDL_GameControllerButton, bool> buttonmap;

desktop_version/src/editor.cpp

@@ -3846,7 +3846,13 @@ void editorinput( KeyPoll& key, Graphics& dwgfx, Game& game, mapclass& map, enti
                 key.keybuffer=ed.sb[ed.pagey+ed.sby];
             }
 
-            if(key.pressedbackspace && ed.sb[ed.pagey+ed.sby]=="")
+            if(key.linealreadyemptykludge)
+            {
+                ed.keydelay=6;
+                key.linealreadyemptykludge=false;
+            }
+
+            if(key.pressedbackspace && ed.sb[ed.pagey+ed.sby]=="" && ed.keydelay<=0)
             {
                 //Remove this line completely
                 ed.removeline(ed.pagey+ed.sby);

desktop_version/src/main.cpp

@@ -473,6 +473,9 @@ int main(int argc, char *argv[])
 
         }
 
+        //We did editorinput, now it's safe to turn this off
+        key.linealreadyemptykludge = false;
+
         if (game.savemystats)
         {
             game.savemystats = false;