Fixes crash on full screen scrolling

[gzcxx]

When using Kakoune(Vim-like editor) to open a file, repeatedly pressing Page Up and Page Down can lead to crashes randomly.
The issue occurs within the fullscreen scrolling branch of vtbackend::Grid::scrollDown.

backtrace:
std::__throw_bad_variant_access (__n=<optimized out>) at /usr/include/c++/15/variant:1420
std::get<...> (__v=std::variant [no contained value]) at /usr/include/c++/15/variant:1153
vtbackend::Line<vtbackend::CompactCell>::inflatedBuffer (this=0x1fa7d50) at contour/src/vtbackend/Line.h:456
vtbackend::Line<vtbackend::CompactCell>::reset (this=0x1fa7d50, flags=..., attributes=...) at contour/src/vtbackend/Line.h:113
                 HERE: "this=0x1fa7d50" is an illegal address!
vtbackend::Grid<vtbackend::CompactCell>::scrollDown (this=this@entry=0x1876448, vN=..., defaultAttributes=..., margin=...) at contour/src/vtbackend/Grid.cpp:529
vtbackend::Screen<vtbackend::CompactCell>::scrollDown (this=this@entry=0x1876380, n=..., margin=...) at contour/src/vtbackend/Screen.cpp:768

Reason:
Grid::rotateBuffersRight(n) performs a rotation on the ring buffer. gsl::span::subspan() cannot correctly handle the ring buffer, so leading to an out-of-bounds access for _lines[].

Solution:
Directly reset the corresponding line.

[Yaraslaut]

Hi, can you please provide more info on how to reproduce this bug ?
Would be nice if we could find a root cause for the incorrect state