Fix saved cursor handling

This resolves several problems with handling of the saved cursor when
switching between primary and alternate screen. Additionally ref-tests
are also added for all common interactions to make sure the behavior
does not regress.

The behavior is based on XTerm's behavior except for interaction with
`reset`. XTerm does not reset the alternate screen's saved cursor on
`reset`, but VTE does. Since a `reset` should reset as much as possible,
Alacritty copies VTE here instead of XTerm.

alacritty_terminal/tests/ref.rs

@@ -64,6 +64,8 @@ ref_tests! {
     clear_underline
     region_scroll_down
     wrapline_alt_toggle
+    saved_cursor
+    saved_cursor_alt
 }
 
 fn read_u8<P>(path: P) -> Vec<u8>

alacritty_terminal/tests/ref/saved_cursor/alacritty.recording

@@ -0,0 +1,8 @@
+[undeadleech@archhq saved_cursor]$ echo -e "\e7 \e(0 test \e8 xxx"
+7 (0 test 8 xxx
+[undeadleech@archhq saved_cursor]$ echo -e "\e[?1049h \e(0 test \e[?1049l xxx"
+[?1049h (0 test [?1049l xxx
+[undeadleech@archhq saved_cursor]$ echo -e "\e7 \e(0 \e[?1049h test \e[?1049l \e8 xxx"
+7 (0 [?1049h test [?1049l 8 xxx
+[undeadleech@archhq saved_cursor]$ exit
+exit

alacritty_terminal/tests/ref/saved_cursor/config.json

@@ -0,0 +1 @@
+{"history_size":0}

alacritty_terminal/tests/ref/saved_cursor/size.json

@@ -0,0 +1 @@
+{"width":1259.0,"height":683.0,"cell_width":9.0,"cell_height":19.0,"padding_x":4.0,"padding_y":9.0,"dpr":1.1666666666666667}

alacritty_terminal/tests/ref/saved_cursor_alt/alacritty.recording

@@ -0,0 +1,4 @@
+[undeadleech@archhq saved_cursor_alt]$ echo -e "\e(0\e[10;$(tput cols)H \e[?1049h\e7 \e[?1049l \e(B\e[H \e[?1049h test\e8xxx"
+(0 [?1049h7 [?1049l (B [?1049h test8xxx
+[undeadleech@archhq saved_cursor_alt]$ exit
+exit

alacritty_terminal/tests/ref/saved_cursor_alt/config.json

@@ -0,0 +1 @@
+{"history_size":0}

alacritty_terminal/tests/ref/saved_cursor_alt/size.json

@@ -0,0 +1 @@
+{"width":1259.0,"height":683.0,"cell_width":9.0,"cell_height":19.0,"padding_x":4.0,"padding_y":9.0,"dpr":1.1666666666666667}