Claw back some internal ram

- 'main' doesn't need 12k of internal ram
 - lvgl's draw task doesn't need that much either
 - also lower the bg worker stack sizes whilst we're here, since they've got tons over headroom

lib/lvgl/src/draw/sw/lv_draw_sw.c

@@ -143,7 +143,7 @@ void lv_draw_sw_init(void)
         draw_sw_unit->base_unit.delete_cb = LV_USE_OS ? lv_draw_sw_delete : NULL;
 
 #if LV_USE_OS
-        lv_thread_init(&draw_sw_unit->thread, LV_THREAD_PRIO_HIGH, render_thread_cb, 8 * 1024, draw_sw_unit);
+        lv_thread_init(&draw_sw_unit->thread, LV_THREAD_PRIO_HIGH, render_thread_cb, 4 * 1024, draw_sw_unit);
 #endif
     }