DRTVWR-588: Enlarge default coroutine stack size.

On a Windows CI host, we got the dreaded rc 3221225725 aka c00000fd aka stack
overflow.

indra/llcommon/llcoros.cpp

@@ -123,7 +123,7 @@ LLCoros::LLCoros():
     // Previously we used
     // boost::context::guarded_stack_allocator::default_stacksize();
     // empirically this is insufficient.
-    mStackSize(768*1024),
+    mStackSize(900*1024),
     // mCurrent does NOT own the current CoroData instance -- it simply
     // points to it. So initialize it with a no-op deleter.
     mCurrent{ [](CoroData*){} }