Enables the SDL driver for LVGL to utilize a user prescribed width/height for display resolution when utilizing SDL, or rely on the SDL_HOR/VER_RES provided by lv_drv_conf.h in lv_drivers
Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
