[nuklear] Added command to enable cursors with default font.

[dlannan]

This adds the cursor to be available for use when using the default font.

To enable cursors in a sokol application:

• First hide the native mouse: sapp_show_mouse(false);
• Then show the nuklear mouse: nk_style_show_cursor(ctx);
  The nuklear show must be called with a valid context.

If not using the default font, then after a font is initialized and added manually call:
nk_style_load_all_cursors(ctx, &your_font_atlas);
And then use the above operations to show the nuklear cursor.

[floooh]

Looks like a nice simple fix (oh, CI fails though, can you look into that?):

MSVC warning:

sokol_nuklear.h(2156,55): warning C4047: 'function': 'nk_cursor *' differs in levels of indirection from 'nk_cursor (*)[7]'

GCC/Clang error:

sokol_nuklear.h:2156:55: error: incompatible pointer types passing 'struct nk_cursor (*)[7]' to parameter of type 'struct nk_cursor *' [-Werror,-Wincompatible-pointer-types]

[dlannan]

@floooh - sorry. I just went through some of these. Will fix. Needs to be:
nk_style_load_all_cursors(&_snuklear.ctx, &_snuklear.atlas.cursors[0]);
Running multi-platform tests now to make sure. Will update as needed.

[floooh]

Thanks for the fixes! The CI error on Android is unrelated, currently investigating...

PS: ok, looks like it was just a hickup, not unusual unfortunately with GH Actions.

I'll test the PR locally a bit and then merge, one sec...

[floooh]

Ok merged. Thanks!

The software mouse cursor actually feels a lot better than expected :) I tested on a 120 Hz display though.

The sapp_show_mouse(false/true) should probably also be hooked up to the SAPP_EVENTTYPE_MOUSE_ENTER/LEAVE events because at least on macOS the system mouse cursor remains hidden when moving it outside the application window (I think that's one of those platform specific behaviour in different window systems).