x11_rawfb: nk_edit_string right border cutoff

I'm attempting to use an adaptation of the x11_rawfb driver for some software-rendered GUI's, and I'm running into trouble where it appears that whatever is drawing the edit string widget is cutting off the right side of the widget. Here is a simple reproducer, done with the original x11_rawfb driver to show it happens with the original.

```c
/* GUI */
if (nk_begin(&rawfb->ctx, "Demo", nk_rect(50, 50, 200, 200),
NK_WINDOW_BORDER|NK_WINDOW_MOVABLE|NK_WINDOW_SCALABLE|
NK_WINDOW_CLOSABLE|NK_WINDOW_MINIMIZABLE|NK_WINDOW_TITLE)) {
char input[64];
int inputlen = 0;
nk_layout_row_dynamic(&rawfb->ctx, 0, 1);
nk_edit_string(&rawfb->ctx, NK_EDIT_SIMPLE, &input[0], &inputlen, 64, nk_filter_default);
}
nk_end(&rawfb->ctx);
```

This results in this output:


I have not extensively tested other widgets.

After examining the draw calls from my app, I have determined that the widget is attempting to draw a rectangle 1px outside of the right scissor. I think that the obvious solution would be to subtract one from the rectangle's x2 and y2 when stroking it, so you end up with a rectangle that fits within the scissor bounds, but I'm unsure what other wider ramifications such a change might have.

Start with the x11_rawfb driver and the provided nk_edit_string reproducer, then inspect the draw calls and scissor bounds around the edit-string widget. The work is done when the widget's right border renders fully within its scissor region without regressing other widgets.

Written by the indexing model from the issue text.