new function in API

[ledinscak]

It would be nice to have function
tb_get_cell(x, y)
of some kind that returns content on specific cell. It is complement to function tb_set_cell(...)

For example I want to render something over background that is dynamic, so I need to get first backround of a cell and then overite ch, but bg should remain same.
I can do it with

  struct tb_cell *cell;
  cellbuf_get(&global.back, x, y, &cell);

but it would be better to hide implementation in my opinion and as I said to have complete API.

[ghost]

https://github.com/termbox/termbox2/blob/master/termbox2.h#L3334
This looks interesting
we could just do the same but with

int tb_get_cell(int x, int y, uint32_t *ch, uintattr_t *fg, uintattr_t *bg);

where the pointers are the data return values and the function's return value is used for error detection.

Currently too lazy to open a PR for this, maybe later :)

[adsr]

PR at #65

There is some overlap with the existing tb_cell_buffer function.

@ledinscak in your case you may want to fetch from the front buffer instead as it will contain the latest rendered cells.

@jumps-are-op To avoid more TB_OPT_EGC ifdefs, my PR uses one tb_cell out param instead of separate out params for ch, fg, bg.