fix: glyph under the block cursor invisible on INVERSE/selected cells

[Patrick-Kappen]

Problem

The block cursor rect is painted in the cell's pre-swap fg color, but the glyph on top of it is recolored to the post-swap bg — and only when APP_CURSOR is set, a keypad/cursor-keys mode that has nothing to do with rendering.

On a cell with the INVERSE flag or inside a selection, the fg/bg swap has already happened by then, so glyph and cursor rect end up the same color: the character under the cursor disappears. Easy to hit in practice:

• nvim: the character under the cursor vanishes (nvim renders its cursor cell inverse)
• selecting across the shell prompt: the letter under the block cursor blinks out

Minimal repro in a plain shell:

printf 'X\e[7mY\e[27mZ\e[2D'; sleep 5   # cursor ends on the inverse Y

Before: X▮Z — the Y is invisible inside the cursor block. After: XYZ with the Y readable inside the block.

Fix

Capture the cell's pre-swap bg and use it for the glyph whenever the cursor is shown on that cell (SHOW_CURSOR), unconditionally — the cursor rect is painted in the pre-swap fg, so the pre-swap bg is the contrasting color regardless of INVERSE/selection. Also drops the unrelated APP_CURSOR gate: without it, the glyph under the cursor was invisible in the (common) non-application-mode case too, whenever cell fg ≈ cursor color.