Alpha Blending in Wayland about lv_port_linux

[symfund]

@kisvegabor In my fork v8.2, displ_drv->screen_transp and LV_COLOR_SCREEN_TRANSP already are enabled, I will try on v8.3.

[kisvegabor]

If you are using LVGL v8.3 you should enable disp_drv->screen_transp and LV_COLOR_SCREEN_TRANSP.

In LVGL master only disp_drv->screen_transp needs to be enabled.

[symfund]

I fixed the problem!

[symfund]

In void lv_disp_drv_init(),

void lv_disp_drv_init(lv_disp_drv_t * driver)
{
...

#if LV_COLOR_SCREEN_TRANSP
driver->screen_transp = 1;
#else
driver->screen_transp = 0;
#endif

...

}