Replace System_SendDebugOutput with a registered callback

I normally try to avoid registrations when not needed, but in this case
only headless uses this, so it's motivated.

ios/main.mm

@@ -664,9 +664,6 @@ AudioBackend *System_CreateAudioBackend() {
 	return nullptr;
 }
 
-bool System_SendDebugOutput(std::string_view data) { return false; }
-void System_SendDebugScreenshot(const uint8_t *data, int width, int height) {}
-
 int main(int argc, char *argv[]) {
 	version = [[[UIDevice currentDevice] systemVersion] UTF8String];
 	if (1 != sscanf(version.c_str(), "%d", &g_iosVersionMajor)) {