Add pure C wayland demo

[skoperst]

This implementation uses software rendering in native wayland client.

[dumblob]

Didn't test it, but overall the PR looks too good to not merge it 😉. Thanks!

I can see there the following TODO items for future contributors:

1. change bubble sort at least to quick sort or better to radix sort (which is the fastest to me known method for sorting integers)
2. in nk_wayland_line_horizontal() there is plenty room for optimization - e.g.:
   1. avoid most of the pointers (especially unsigned int *ptr working with the same memory as unsigned char *pixels)
   2. pass win->data and win->width directly instead of just win
   3. use uint8_t everywhere instead of unsigned int
   4. use restrict for all pointers
   5. manually unroll first two loops (C macros could ease readability)
   6. make it inline
3. nk_wayland_stroke_line() could also get some optimization love similar to (2) 😉
4. there are some unimplemented NK_COMMAND_...