Do not panic in Application::try_global if main thread is unclaimed

`Application::try_global` called `assert_main_thread` but before
Application is running the main thread is unclaimed. I think there was
no way that `Application::try_global` to return None.

Added `assert_main_thread_or_main_unclaimed` that does not panic
if main thread is unclaimed.

druid-shell/src/application.rs

@@ -121,7 +121,7 @@ impl Application {
     /// [`new`]: #method.new
     /// [`run`]: #method.run
     pub fn try_global() -> Option<Application> {
-        util::assert_main_thread();
+        util::assert_main_thread_or_main_unclaimed();
         GLOBAL_APP.with(|global_app| global_app.borrow().clone())
     }
 

druid-shell/src/util.rs

@@ -32,6 +32,7 @@ fn current_thread_id() -> u64 {
 /// # Panics
 ///
 /// Panics when called from a non-main thread.
+#[allow(dead_code)]
 pub fn assert_main_thread() {
     let thread_id = current_thread_id();
     let main_thread_id = MAIN_THREAD_ID.load(Ordering::Acquire);
@@ -43,6 +44,22 @@ pub fn assert_main_thread() {
     }
 }
 
+/// Assert that the current thread is the registered main thread or main thread is not claimed.
+///
+/// # Panics
+///
+/// Panics when called from a non-main thread and main thread is claimed.
+pub fn assert_main_thread_or_main_unclaimed() {
+    let thread_id = current_thread_id();
+    let main_thread_id = MAIN_THREAD_ID.load(Ordering::Acquire);
+    if thread_id != main_thread_id && main_thread_id != 0 {
+        panic!(
+            "Main thread assertion failed {} != {}",
+            thread_id, main_thread_id
+        );
+    }
+}
+
 /// Register the current thread as the main thread.
 ///
 /// # Panics