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.
Fixed that.

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

terminal-emulator/src/main/java/com/termux/terminal/TerminalEmulator.java

@@ -1730,8 +1730,8 @@ private void doCsi(int b) {
  final int linesToScrollArg = getArg0(1);
  final int linesBetweenTopAndBottomMargins = mBottomMargin - mTopMargin;
  final int linesToScroll = Math.min(linesBetweenTopAndBottomMargins, linesToScrollArg);
- mScreen.blockCopy(0, mTopMargin, mColumns, linesBetweenTopAndBottomMargins - linesToScroll, 0, mTopMargin + linesToScroll);
- blockClear(0, mTopMargin, mColumns, linesToScroll);
+ mScreen.blockCopy(mLeftMargin, mTopMargin, mRightMargin - mLeftMargin, linesBetweenTopAndBottomMargins - linesToScroll, mLeftMargin, mTopMargin + linesToScroll);
+ blockClear(mLeftMargin, mTopMargin, mRightMargin - mLeftMargin, linesToScroll);
  } else {
  // "${CSI}${func};${startx};${starty};${firstrow};${lastrow}T" - initiate highlight mouse tracking.
  unimplementedSequence(b);