Metal: align the caret to the rounded glyph cell grid (fixes cursor drift that grows with column)

[kriera]

Problem

On the Metal renderer, the text cursor drifts to the right of the glyphs, and the offset grows with the column — invisible near the left edge, a visible multi-cell gap at a long prompt (e.g. a ~30-column shell prompt leaves ~1–2 empty cells between the prompt and the cursor / typed input). The CoreGraphics renderer is unaffected.

Root cause

Glyphs and the caret are placed on different horizontal grids:

• Glyphs (buildRowData) snap to a rounded, pixel-aligned grid:
  x = round(lineOriginPx.x) + column * round(cellWidthPx)
• The caret (buildCursorDrawData) used the unrounded fractional grid:
  x = lineOriginPx.x + buffer.x * cellWidthPx

So the caret diverges from the glyphs by round(lineOriginPx.x) - lineOriginPx.x plus buffer.x * (round(cellWidthPx) - cellWidthPx) — a per-column error that accumulates with buffer.x.

Fix

Position the caret on the same rounded/aligned grid the glyphs use (baseCellWidthPx + alignedOriginX), preserving the existing doublePosition and cursorColumnWidth (full-width/CJK) behavior:

let baseCellWidthPx = CGFloat(max(1, Int(round(cellWidthPx))))
let alignedOriginX  = round(lineOriginPx.x)
...
let x0 = alignedOriginX + CGFloat(buffer.x) * baseCellWidthPx * doublePosition
let x1 = x0 + baseCellWidthPx * doublePosition * cursorColumnWidth

Note (related, optional)

The same fractional-vs-rounded inconsistency exists for background/decoration cells in buildRowData (they use column * cellWidthPx), which can make the cell highlight / underline drift from the glyph on long lines too. I kept this PR surgical (caret only) since that's the most visible symptom — happy to fold those into the same rounded grid here or in a follow-up.

Repro

Retina Mac, Metal renderer on, any shell with a ~30-column prompt (e.g. conda's (base) …). Type a character: it appears ~1–2 cells right of the prompt's end; the gap scales with prompt length. Off on CoreGraphics. After the patch: the caret sits tight against the prompt at all columns.

swift build passes.

[migueldeicaza]

This looks like the same case in #615 which was fixed in 13732b7 can you check that please?