[Core] Experimental Core API (?)

[filippocrocchini]

Ignore the CLAY_NAMED stuff, that is a duplicate implementation of #492 and will get removed once it's merged.

In my experience rolling out my own immediate mode UI libraries, I have gravitated towards having a core layout engine which you can hook into to do things at different times in the pipeline. Be it rendering, input and whatnot.

I don't expect this to be merged in as it's incomplete first of all, but I believe this kind of API could allow the debug visualization, rendering, input and more to be separated from the core. I believe you are working towards that and here are my two cents on that.

I would love feedback on this, especially regarding how it would interact with floating elements. I am almost 100% sure that, as it is, it won't work in some corner cases.

Stripped down input example:

    void process_input_on_element(void* userData, uint32_t id, Clay_ElementData data, Clay_LayoutConfig* layout, Clay_ElementState state)
    {
        auto visible_region = data.visibleBoundingBox;

        if (state == CLAY_ELEMENT_STATE_OPEN)
        {
            if (contains(visible_region, cursor_position)) {
                if (mouse_owner == id && left_button_down)
                {
                    pressed_id  = id;
                }
                if (mouse_owner == id && left_button_just_released)
                {
                    clicked_id  = id;
                    mouse_owner = 0;
                }
            }
        }

        // Here we do it on end finalize, because we want to give ownership of the input to the deepest node that is
        // currently hovered. This doesn't always work especially with floating elements. However, if we had a zIndex
        // for every element then it should work. 
        
        if (state == CLAY_ELEMENT_STATE_END_FINALIZE)
        {
            if (contains(visible_region, cursor_position))
            {
                if (left_button_just_pressed && !new_mouse_owner && is_clickable(id))
                {
                    new_mouse_owner = id;
                    mouse_owner     = id;
                }
            }
        }
    }