Issue with scrolling up within a scrollable region

[liamg]

Scrolling up within a scrollable region in certain applications simply does not work. The line the cursor is on does update and scrolls content, but all other lines remain stationary and not updated.

To Reproduce
Steps to reproduce the behavior:

1. Run htop
2. Hold the down arrow and scroll to the end of the list of processes (you may need to shrink the window to make the list become scrollable)
3. Hold the up arrow and scroll back up. When the cursor reaches the top line of the scrollable region (list of processes), the list should begin to scroll, but it does not.

Expected behavior
In step 3 above, the list should scroll back toward the top.

Additional context
I think the origin mode being implemented may fix this issue? See https://vt100.net/docs/vt510-rm/DECOM.html

[liamg]

Dump of CSI codes received from htop after single scroll up with up arrow:

2018-10-22T13:54:48.762+0100	DEBUG	gui/input.go:23	KEY PRESS: key=0x109 scan=0x6F
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x72 (ESC[9;42r) Set Scrolling Region [top;bottom] (default = full size of window) (DECSTBM), VT100
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x48 (ESC[9;1H) Cursor Position [row;column] (default = [1,1]) (CUP)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x72 (ESC[1;43r) Set Scrolling Region [top;bottom] (default = full size of window) (DECSTBM), VT100
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x48 (ESC[9;1H) Cursor Position [row;column] (default = [1,1]) (CUP)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[30m) Character Attributes (SGR)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[46m) Character Attributes (SGR)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x4B (ESC[K) Erase in Line (EL), VT100
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x64 (ESC[10d) Line Position Absolute  [row] (default = [1,column]) (VPA)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[39;49m) Character Attributes (SGR)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[m) Character Attributes (SGR)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[0;1m) Character Attributes (SGR)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[90m) Character Attributes (SGR)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[39m) Character Attributes (SGR)
2018-10-22T13:54:48.764+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[m) Character Attributes (SGR)
2018-10-22T13:54:48.765+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[32m) Character Attributes (SGR)
2018-10-22T13:54:48.765+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[39m) Character Attributes (SGR)
2018-10-22T13:54:48.765+0100	DEBUG	terminal/csi.go:78	CSI 0x6D (ESC[m) Character Attributes (SGR)
2018-10-22T13:54:48.765+0100	DEBUG	terminal/csi.go:78	CSI 0x4B (ESC[K) Erase in Line (EL), VT100
2018-10-22T13:54:48.765+0100	DEBUG	terminal/csi.go:78	CSI 0x48 (ESC[H) Cursor Position [row;column] (default = [1,1]) (CUP)

[liamg]

Fixed in #20