Fix cursor disappear and rendering issue with Chinese characters

[bommbo]

When switching between Block and I-beam cursor shapes on wide characters (e.g. Chinese), the cursor area was too narrow due to using _fontWidth instead of the actual character pixel width. This caused visual artifacts
such as partial cursor remnants or incomplete redraw.

This change uses QFontMetrics::horizontalAdvance() to get the true width of the character under the cursor, while preserving the existing -1/+1 adjustment to maintain I-beam visibility under sub-pixel rendering.

[bommbo]

qterminal

before

after

[tsujan]

First, thanks for your contribution!

Chinese characters come under the "double-column" category. IMO, there's no issue to fix: the blinking cursor is easy to recognize, although its behavior is different with double-column characters.

Moreover:

• I doubt that doing extra computations is a good idea in the case of double-column characters when it comes to blinking.
• This patch may cause inconsistencies in the case of other characters that are NOT double-column. ⧸ (U+29F8 — not an ordinary slash) is only one example.