Clip character drawing to its own cell

Summary:
To calculate string width, Konsole uses character cell widths.
Rendering engine uses actual characters widths for this purpose.
Those sizes are equal in fixed-width fonts, as long as the font
has specific character. Since no font contains all Unicode characters,
some of them are taken from different fonts and their sizes differ
from their cell size. This can lead to different results depending
on how text line is split into strings.

It can be assumed that characters in Basic Latin block in every
fixed-width font are really fixed-width, and can be rendered as one
string. All other characters are rendered separately, and their
rendering surface is limited to their cell.

Before/After screenshots:
{F5749612}

BUG: 361547

Test Plan:
Paste this: `⸻test` in konsole and highlight different parts of the string

In a text editor paste this:
```
| A |
| ⸻ |
| ⟹ |
| ⧠ |
```
and move cursor vertically through characters column

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, ngraham, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D11237

src/TerminalDisplay.cpp

@@ -940,11 +940,13 @@ void TerminalDisplay::drawCharacters(QPainter& painter,
         // This still allows RTL characters to be rendered in the RTL way.
         painter.setLayoutDirection(Qt::LeftToRight);
 
+        painter.setClipRect(rect);
         if (_bidiEnabled) {
             painter.drawText(rect.x(), rect.y() + _fontAscent + _lineSpacing, text);
         } else {
             painter.drawText(rect.x(), rect.y() + _fontAscent + _lineSpacing, LTR_OVERRIDE_CHAR + text);
         }
+        painter.setClipping(false);
     }
 }
 
@@ -1631,39 +1633,42 @@ void TerminalDisplay::drawContents(QPainter& paint, const QRect& rect)
             const CharacterColor currentBackground = _image[loc(x, y)].backgroundColor;
             const RenditionFlags currentRendition = _image[loc(x, y)].rendition;
 
-            while (x + len <= rlx &&
-                    _image[loc(x + len, y)].foregroundColor == currentForeground &&
-                    _image[loc(x + len, y)].backgroundColor == currentBackground &&
-                    (_image[loc(x + len, y)].rendition & ~RE_EXTENDED_CHAR) == (currentRendition & ~RE_EXTENDED_CHAR) &&
-                    (_image[ qMin(loc(x + len, y) + 1, _imageSize) ].character == 0) == doubleWidth &&
-                    _image[loc(x + len, y)].isLineChar() == lineDraw) {
-                const quint16 c = _image[loc(x + len, y)].character;
-                if ((_image[loc(x + len, y)].rendition & RE_EXTENDED_CHAR) != 0) {
-                    // sequence of characters
-                    ushort extendedCharLength = 0;
-                    const ushort* chars = ExtendedCharTable::instance.lookupExtendedChar(c, extendedCharLength);
-                    if (chars != nullptr) {
-                        Q_ASSERT(extendedCharLength > 1);
-                        bufferSize += extendedCharLength - 1;
-                        unistr.resize(bufferSize);
-                        disstrU = unistr.data();
-                        for (int index = 0 ; index < extendedCharLength ; index++) {
+            if(_image[loc(x, y)].character <= 0x7e) {
+                while (x + len <= rlx &&
+                        _image[loc(x + len, y)].foregroundColor == currentForeground &&
+                        _image[loc(x + len, y)].backgroundColor == currentBackground &&
+                        (_image[loc(x + len, y)].rendition & ~RE_EXTENDED_CHAR) == (currentRendition & ~RE_EXTENDED_CHAR) &&
+                        (_image[ qMin(loc(x + len, y) + 1, _imageSize) ].character == 0) == doubleWidth &&
+                        _image[loc(x + len, y)].isLineChar() == lineDraw &&
+                        _image[loc(x + len, y)].character <= 0x7e) {
+                    const quint16 c = _image[loc(x + len, y)].character;
+                    if ((_image[loc(x + len, y)].rendition & RE_EXTENDED_CHAR) != 0) {
+                        // sequence of characters
+                        ushort extendedCharLength = 0;
+                        const ushort* chars = ExtendedCharTable::instance.lookupExtendedChar(c, extendedCharLength);
+                        if (chars != nullptr) {
+                            Q_ASSERT(extendedCharLength > 1);
+                            bufferSize += extendedCharLength - 1;
+                            unistr.resize(bufferSize);
+                            disstrU = unistr.data();
+                            for (int index = 0 ; index < extendedCharLength ; index++) {
+                                Q_ASSERT(p < bufferSize);
+                                disstrU[p++] = chars[index];
+                            }
+                        }
+                    } else {
+                        // single character
+                        if (c != 0u) {
                             Q_ASSERT(p < bufferSize);
-                            disstrU[p++] = chars[index];
+                            disstrU[p++] = c; //fontMap(c);
                         }
                     }
-                } else {
-                    // single character
-                    if (c != 0u) {
-                        Q_ASSERT(p < bufferSize);
-                        disstrU[p++] = c; //fontMap(c);
-                    }
-                }
 
-                if (doubleWidth) { // assert((_image[loc(x+len,y)+1].character == 0)), see above if condition
-                    len++; // Skip trailing part of multi-column character
+                    if (doubleWidth) { // assert((_image[loc(x+len,y)+1].character == 0)), see above if condition
+                        len++; // Skip trailing part of multi-column character
+                    }
+                    len++;
                 }
-                len++;
             }
             if ((x + len < _usedColumns) && (_image[loc(x + len, y)].character == 0u)) {
                 len++; // Adjust for trailing part of multi-column character