Fix block cursor to render text with cursorAccent color

[ncoop57]

Problem

The cursorAccent theme property was defined in xtermjs' ITheme but never actually used. Block cursors rendered as opaque rectangles that completely cover the character underneath, making it invisible.

Solution

• Added optional colorOverride parameter to renderCellText()
• After drawing the block cursor, re-render the cell's text using cursorAccent as the color override

This is a minimal change that reuses the existing text rendering logic, ensuring proper handling of font styles, graphemes, and wide characters.

PR created with the assistance of SolveIT, manually reviewed, iterated, and tested personally.

[deansheather]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid repainting full-width glyph outside block cursor cell

Redrawing line[x] via renderCellText can repaint a double-width character (cell.width === 2) across both cells, but the block cursor background drawn just above still covers only one cell. In the default theme cursorAccent is the same as the background, so when the cursor lands on a full-width CJK glyph, the right half gets repainted in background color on the non-cursor cell and effectively disappears. This regression is introduced by the new redraw path and is reproducible whenever block cursor is positioned on wide characters.

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