fix(display): POSIX lvgl fixes (#2812)

fix(display): Do LVGL task processing in main on POSIX.

An SDL/Zephyr bug prevents proper display when SDL calls are
made from anything but the main thread, so add task handling
in our simple main function when on POSIX.

fix(usb): Compilation fix for 64-bit targets

Properly handle differences in the size of `size_t` on 64-bit
architectures.

fix(display): Imply, but don't force, LVGL mono theme

Some targets may be using color displays, so instead of forcing
on the mono theme, merely imply it to default it

app/src/main.c

@@ -12,7 +12,12 @@
 #include <zephyr/logging/log.h>
 LOG_MODULE_REGISTER(zmk, CONFIG_ZMK_LOG_LEVEL);
 
+#if IS_ENABLED(CONFIG_ZMK_DISPLAY)
+
 #include <zmk/display.h>
+#include <lvgl.h>
+
+#endif
 
 int main(void) {
     LOG_INF("Welcome to ZMK!\n");
@@ -24,6 +29,16 @@ int main(void) {
 
 #ifdef CONFIG_ZMK_DISPLAY
     zmk_display_init();
+
+#if IS_ENABLED(CONFIG_ARCH_POSIX)
+    // Workaround for an SDL display issue:
+    // https://github.com/zephyrproject-rtos/zephyr/issues/71410
+    while (1) {
+        lv_task_handler();
+        k_sleep(K_MSEC(10));
+    }
+#endif
+
 #endif /* CONFIG_ZMK_DISPLAY */
 
     return 0;