TerminalDisplay: Limit mouse move events.

The terminal display mouse tracking is based on character boxes, i.e.,
unless the mouse moves out of a character box its position hasn't really
changed. So most of the logic in `TerminalDisplay::mouseMoveEvent` can
be skipped/omitted till the pointer crosses box boundaries.

Before my changes applications like Vim/Neovim couldn't do selection via
dragging with the left mouse button (there might be a bug there). I
cursorily checked other features like the color filter and URL
underlining, and they work.

src/terminalDisplay/TerminalDisplay.cpp

@@ -355,6 +355,8 @@ TerminalDisplay::TerminalDisplay(QWidget* parent)
     , _boldIntense(true)
     , _lines(1)
     , _columns(1)
+    , _prevCharacterLine(-1)
+    , _prevCharacterColumn(-1)
     , _usedLines(1)
     , _usedColumns(1)
     , _contentRect(QRect())
@@ -1413,6 +1415,13 @@ void TerminalDisplay::mouseMoveEvent(QMouseEvent* ev)
 
     auto [charLine, charColumn] = getCharacterPosition(ev->pos(), !_usesMouseTracking);
 
+    if (charLine == _prevCharacterLine && charColumn == _prevCharacterColumn) {
+        return;
+    }
+
+    _prevCharacterLine = charLine;
+    _prevCharacterColumn = charColumn;
+
     processFilters();
 
     _filterChain->mouseMoveEvent(this, ev, charLine, charColumn);
@@ -1422,7 +1431,7 @@ void TerminalDisplay::mouseMoveEvent(QMouseEvent* ev)
     }
 
     // if the program running in the terminal is interested in Mouse Tracking
-    // evnets then emit a mouse movement signal, unless the shift key is
+    // events then emit a mouse movement signal, unless the shift key is
     // being held down, which overrides this.
     if (_usesMouseTracking && !(ev->modifiers() & Qt::ShiftModifier)) {
         int button = 3;

src/terminalDisplay/TerminalDisplay.h

@@ -684,6 +684,11 @@ private:
     int _lines;      // the number of lines that can be displayed in the widget
     int _columns;    // the number of columns that can be displayed in the widget
 
+    // Character line and character column as per a previous call to
+    // getCharacterPosition() in mouseMoveEvent().
+    int _prevCharacterLine;
+    int _prevCharacterColumn;
+
     int _usedLines;  // the number of lines that are actually being used, this will be less
     // than 'lines' if the character image provided with setImage() is smaller
     // than the maximum image size which can be displayed