Add os_thread_attach/os_thread_detach;

These are called when creating/destroying Thread objects.  It's
currently only implemented on Android, where it attaches/detaches the
Java VM to the thread.  This allows JNI calls to be used on threads.

However I don't think e.g. `lovr.system.requestPermission` will work on
a thread yet, because it uses the global JNI env from the main thread
instead of the thread's JNI env.  Still, attaching/detaching the VM is
an improvement and will allow well-behaved JNI methods to work on
threads now.

I don't know how expensive this is, yolo.

src/core/os.h

@@ -149,6 +149,9 @@ bool os_vm_free(void* p, size_t size);
 bool os_vm_commit(void* p, size_t size);
 bool os_vm_release(void* p, size_t size);
 
+void os_thread_attach(void);
+void os_thread_detach(void);
+
 void os_poll_events(void);
 void os_on_quit(fn_quit* callback);
 void os_on_focus(fn_focus* callback);

src/core/os_android.c

@@ -281,6 +281,15 @@ bool os_vm_release(void* p, size_t size) {
   return !madvise(p, size, MADV_DONTNEED);
 }
 
+void os_thread_attach(void) {
+  JNIEnv* jni;
+  (*state.app->activity->vm)->AttachCurrentThread(state.app->activity->vm, &jni, NULL);
+}
+
+void os_thread_detach(void) {
+  (*state.app->activity->vm)->DetachCurrentThread(state.app->activity->vm);
+}
+
 // Notes about polling:
 // - Stop polling if a destroy is requested to give the application a chance to shut down.
 //   Otherwise this loop would still wait for an event and the app would seem unresponsive.

src/core/os_linux.c

@@ -64,6 +64,14 @@ bool os_vm_release(void* p, size_t size) {
   return !madvise(p, size, MADV_DONTNEED);
 }
 
+void os_thread_attach(void) {
+  //
+}
+
+void os_thread_detach(void) {
+  //
+}
+
 void os_window_message_box(const char* message) {
   //
 }

src/core/os_macos.c

@@ -105,6 +105,14 @@ void os_on_permission(fn_permission* callback) {
   state.onPermissionEvent = callback;
 }
 
+void os_thread_attach(void) {
+  //
+}
+
+void os_thread_detach(void) {
+  //
+}
+
 void os_window_message_box(const char* message) {
   //
 }

src/core/os_wasm.c

@@ -246,6 +246,14 @@ void os_request_permission(os_permission permission) {
   //
 }
 
+void os_thread_attach(void) {
+  //
+}
+
+void os_thread_detach(void) {
+  //
+}
+
 void os_poll_events(void) {
   //
 }

src/core/os_win32.c

@@ -116,6 +116,14 @@ bool os_vm_release(void* p, size_t size) {
   return VirtualFree(p, 0, MEM_DECOMMIT);
 }
 
+void os_thread_attach(void) {
+  //
+}
+
+void os_thread_detach(void) {
+  //
+}
+
 void os_on_permission(fn_permission* callback) {
   //
 }

src/modules/thread/thread.c

@@ -1,6 +1,7 @@
 #include "thread/thread.h"
 #include "data/blob.h"
 #include "event/event.h"
+#include "core/os.h"
 #include "util.h"
 #include "lib/tinycthread/tinycthread.h"
 #include <math.h>
@@ -78,8 +79,12 @@ Channel* lovrThreadGetChannel(const char* name) {
 static int threadFunction(void* data) {
   Thread* thread = data;
 
+  os_thread_attach();
+
   char* error = thread->function(thread, thread->body, thread->arguments, thread->argumentCount);
 
+  os_thread_detach();
+
   mtx_lock(&thread->lock);
   thread->running = false;
   if (error) {