fix(vt): clamp scroll margins to screen bounds

[trentkm]

Out-of-range DECSTBM and DECSLRM trailing margins were stored beyond the render buffer. A later reverse-index (ESC M) passed that rectangle to ultraviolet and panicked while indexing the first cell outside the grid.

Clamp bottom and right margins to the current screen dimensions, while preserving the existing handling of one-cell regions at the lower/right edge. Reject leading margins that begin beyond the screen.

This was observed in a live terminal resize and reduces to:

term := vt.NewEmulator(30, 4)
term.Write([]byte("\x1b[?69h\x1b[1;31s\x1b[H\x1bM"))

Tests cover both DECSTBM and DECSLRM bounds.

Tested with go test ./... from vt/.

Downstream report: trentkm/stormlight#199