Fixed: Make ScrollDown escape respect margins

SD sequence (`${CSI}${N}T`) was scrolling the whole width
 of the terminal instead of just between the margins.
RI sequence (`${ESC}M`, move cursor up 1 line) was doing the same.
Fixed that.

Fixes #2576 where in tmux scrolling one of several
 side-by-side panels down resulted in all visually scrolling.

terminal-emulator/src/test/java/com/termux/terminal/ScrollRegionTest.java

@@ -75,6 +75,16 @@ public void testNELRespectsLeftMargin() {
 		withTerminalSized(3, 3).enterString("\033[?69h\033[2sABC\033[?6h\033ED").assertLinesAre("ABC", " D ", "   ");
 	}
 
+	public void testRiRespectsLeftMargin() {
+		// Reverse Index (RI), ${ESC}M, should respect horizontal margins:
+		withTerminalSized(4, 3).enterString("ABCD\033[?69h\033[2;3s\033[?6h\033M").assertLinesAre("A  D", " BC ", "    ");
+	}
+
+	public void testSdRespectsLeftMargin() {
+		// Scroll Down (SD), ${CSI}${N}T, should respect horizontal margins:
+		withTerminalSized(4, 3).enterString("ABCD\033[?69h\033[2;3s\033[?6h\033[2T").assertLinesAre("A  D", "    ", " BC ");
+	}
+
 	public void testBackwardIndex() {
 		// vttest "Menu 11.3.2: VT420 Cursor-Movement Test", test 7.
 		// Without margins: