In some case, when the clip is, eg on a 0.25 of a pixel, but the offset is 0.75,
(such that the begining of the glyph starts at 0.5) difference in
rounding in different computation can result in a width which is one
pixel bigger than the source image.
I couldn't reproduce this in a testcase, but this can be reproduced
while resizing or scrolling through a TextEdit

I think this fixes #2957

The panic fixed by this commit looks like this:
```
thread 'main' panicked at 'index out of bounds: the len is 131 but the index is 131', /home/olivier/slint/internal/core/software_renderer/draw_functions.rs:81:19
stack backtrace:
   3: i_slint_core::software_renderer::draw_functions::draw_texture_line
             at ./internal/core/software_renderer/draw_functions.rs:81:19
   4: <i_slint_core::software_renderer::RenderToBuffer<T> as i_slint_core::software_renderer::ProcessScene>::process_shared_image_buffer
             at ./internal/core/software_renderer.rs:1023:13
   5: i_slint_core::software_renderer::SceneBuilder<T>::draw_text_paragraph::{{closure}}
             at ./internal/core/software_renderer.rs:1404:37
   6: i_slint_core::textlayout::TextParagraphLayout<Font>::layout_lines::{{closure}}
             at ./internal/core/textlayout.rs:219:17
```