fix regression on windows

Author: wjakob

src/lock.h

@@ -28,7 +28,7 @@
 struct Lock {
     pthread_spinlock_t lock;
     pthread_t owner;
-    int recursion_count;
+    size_t recursion_count;
 };
 
 // Danger zone: the drjit-core locks are held for an extremely short amount of
@@ -63,7 +63,7 @@ inline void lock_release(Lock &lock) {
 struct Lock {
     os_unfair_lock_s lock;
     pthread_t owner;
-    int recursion_count;
+    size_t recursion_count;
 };
 
 inline void lock_init(Lock &lock) {
@@ -91,26 +91,25 @@ inline void lock_release(Lock &lock) {
     }
 }
 #else
+#include <thread>
 #if defined(_WIN32)
 #include <shared_mutex>
-#include <thread>
 struct Lock {
     std::shared_mutex lock; // Based on the faster Win7 SRWLOCK
     std::thread::id owner;
-    int recursion_count;
+    size_t recursion_count;
 };
 #else
 #include <mutex>
-#include <thread>
 struct Lock {
     std::mutex lock;
     std::thread::id owner;
-    int recursion_count;
+    size_t recursion_count;
 };
 #endif
 
 inline void lock_init(Lock &lock) {
-    lock.owner           = std::thread::id(-1);
+    lock.owner           = std::thread::id();
     lock.recursion_count = 0;
 }
 
@@ -129,7 +128,7 @@ inline void lock_acquire(Lock &lock) {
 inline void lock_release(Lock &lock) {
     lock.recursion_count--;
     if (lock.recursion_count == 0) {
-        lock.owner = std::thread::id(-1);
+        lock.owner = std::thread::id();
         lock.lock.unlock();
     }
 }