Add function to get disp->rendering_in_progress state.

[spbgzh]

LVGL version

v9.1

What happened?

To avoid Asserted at expression: !disp->rendering_in_progress (Invalidate area is not allowed during rendering.), I need to check rendering_in_progress state, but it in lv_display_private.h, if I want to access it without "lvgl/src/display/lv_display_private.h", it will show error: invalid use of incomplete.
so in this case I suggest to set a globle function to get rendering_in_progress state

How to reproduce?

#include "lvgl/src/display/lv_display_private.h"
static uint8_t perform_screen_transition(lv_screen_to_t *transition)
{
if (disp->rendering_in_progress)
{
return 0;
}
if (transition == NULL || transition->next == NULL)
return 0;
if (transition->next->screen == NULL && transition->next->init != NULL)
transition->next->init();
if (transition->next->screen != NULL)
{
lv_screen_load_anim(transition->next->screen, transition->anim, transition->speed, transition->delay, false);
node = *(transition->next);
return 1;
}
return 0;
}

[spbgzh]

I made a PR #6448