Darwin: Schedule isochronous transfers further in the future

This is required to make OpenKinect transfer data at 30 fps. This
change was originally made by Theo Watson, and Thomas improved on it.
The commit came via OpenKinect into libusb thanks to Kyle Machulis and
Hector Martin. Fixes #66.

libusb/os/darwin_usb.c

diff --git a/libusb/os/darwin_usb.c b/libusb/os/darwin_usb.c
index 9121dd1..169fa1a 100644
--- a/libusb/os/darwin_usb.c
+++ b/libusb/os/darwin_usb.c
@@ -1219,7 +1219,10 @@
   }
 
   /* schedule for a frame a little in the future */
-  frame += 2;
+  frame += 4;
+
+  if (cInterface->frames[transfer->endpoint] && frame < cInterface->frames[transfer->endpoint])
+    frame = cInterface->frames[transfer->endpoint];
 
   /* submit the request */
   if (is_read)
@@ -1231,6 +1234,8 @@
 							      transfer->num_iso_packets, tpriv->isoc_framelist, darwin_async_io_callback,
 							      itransfer);
 
+  cInterface->frames[transfer->endpoint] = frame + transfer->num_iso_packets / 8;
+
   if (kresult != kIOReturnSuccess) {
     usbi_err (TRANSFER_CTX (transfer), "isochronous transfer failed (dir: %s): %s", is_read ? "In" : "Out",
 	       darwin_error_str(kresult));

libusb/os/darwin_usb.h

diff --git a/libusb/os/darwin_usb.h b/libusb/os/darwin_usb.h
index dc14ed6..9ffd6df 100644
--- a/libusb/os/darwin_usb.h
+++ b/libusb/os/darwin_usb.h
@@ -139,6 +139,7 @@
     usb_interface_t    **interface;
     uint8_t              num_endpoints;
     CFRunLoopSourceRef   cfSource;
+    uint64_t             frames[256];
     uint8_t            endpoint_addrs[USB_MAXENDPOINTS];
   } interfaces[USB_MAXINTERFACES];
 };