Terminal scrollback with ring buffer

(apply st-scrollback-ringbuffer-0.9.2.diff)

from the patch header:

commit 0663bdf11a409961da5b1120741a69814da8ce65
Author: Timo Röhling <timo@gaussglocke.de>
Date:   Tue Nov 23 19:45:33 2021 +0100

    Terminal scrollback with ring buffer

    This patch adds a ring buffer for scrollback to the terminal.  The
    advantage of using a ring buffer is that the common case, scrolling with
    no static screen content, can be achieved very efficiently by
    incrementing and decrementing the starting line (modulo buffer size).

    The scrollback buffer is limited to HISTSIZE lines in order to bound
    memory usage. As the lines are allocated on demand, it is possible to
    implement unlimited scrollback with few changes.  If the terminal is
    reset, the scroll back buffer is reset, too.

config.def.h

@@ -253,6 +253,8 @@ static Shortcut shortcuts[] = {
 	{ MODKEY,               XK_9,           selectscheme,   {.i =  8} },
 	{ MODKEY,               XK_0,           nextscheme,     {.i = +1} },
 	{ MODKEY|ControlMask,   XK_0,           nextscheme,     {.i = -1} },
+	{ ShiftMask,            XK_Page_Up,     kscrollup,      {.i = -1} },
+	{ ShiftMask,            XK_Page_Down,   kscrolldown,    {.i = -1} },
 };
 
 /*