xlib: Images cause scrollable content to break

Nobody has claimed this yet.

In the pure x11 demo, after enabling stb_image and loading your images via the `nk_xsurf_load_image_from_file` function

You then use that loaded image later on in for example `nk_button_image_label(ctx, icon, "text1", NK_TEXT_CENTERED);`

However if any content in that window is scrollable it causes weird rendering/clipping issues.

Here it is when there is scrolling


And when there is no scrolling


This issue goes away if I forgo the image/icon by doing `nk_button_label(ctx, "text1");` instead, but that kinda defeats the whole point of having image loading support.

To reproduce my examples add the following to `demo/x11/main.c`
line 17:
`#define NK_XLIB_INCLUDE_STB_IMAGE`
`#define NK_XLIB_IMPLEMENT_STB_IMAGE`

line 154:
```
// Make sure the is a image file named 'image.png' in the directory you launch the demo
struct nk_image icon = nk_xsurf_load_image_from_file("image.png");
```
line 177:
```
nk_layout_row_dynamic(ctx, 64, 3);
nk_button_image_label(ctx, icon, "text1", NK_TEXT_CENTERED);
nk_button_image_label(ctx, icon, "text1", NK_TEXT_CENTERED);
nk_button_image_label(ctx, icon, "text1", NK_TEXT_CENTERED);
```
I believe I also had change the makefile to use C99 instead of C89 so that stb_image would compile.

Please let me know if I'm doing anything, wrong or if you guys need more info.

Start with demo/x11/main.c and reproduce the issue using NK_XLIB_INCLUDE_STB_IMAGE, NK_XLIB_IMPLEMENT_STB_IMAGE, nk_xsurf_load_image_from_file, and nk_button_image_label. Compare the rendering with and without scrolling, then trace the image path involved in those entry points. Done means image buttons render correctly in scrollable and non-scrollable content.

Written by the indexing model from the issue text.