PR Introduces GPU_storagebuf_sync_to_host as an explicit routine to
flush GPU-resident storage buffer memory back to the host within the
GPU command stream.

The previous implmentation relied on implicit synchronization of
resources using OpenGL barriers which does not match the
paradigm of explicit APIs, where indiviaul resources may need
to be tracked.

This patch ensures GPU_storagebuf_read can be called without
stalling the GPU pipeline while work finishes executing. There are
two possible use cases:

1) If GPU_storagebuf_read is called AFTER an explicit call to
GPU_storagebuf_sync_to_host, the read will be synchronized.
If the dependent work is still executing on the GPU, the host
will stall until GPU work has completed and results are available.

2) If GPU_storagebuf_read is called WITHOUT an explicit call to
GPU_storagebuf_sync_to_host, the read will be asynchronous
and whatever memory is visible to the host at that time will be used.
(This is the same as assuming a sync event has already been signalled.)

This patch also addresses a gap in the Metal implementation where
there was missing read support for GPU-only storage buffers.
This routine now uses a staging buffer to copy results if no
host-visible buffer was available.

Reading from a GPU-only storage buffer will always stall
the host, as it is not possible to pre-flush results, as no
host-resident buffer is available.

Authored by Apple: Michael Parkin-White

@@ -1328,6 +1328,8 @@ void ShadowModule::set_view(View &view)
 
       shadow_multi_view_.compute_procedural_bounds();
 
+      statistics_buf_.current().sync_to_host();

@@ -55,0 +65,4 @@
+ *
+ * Otherwise, this command is unsynchronized and will return current visible storage buffer
+ * contents immediately.
+ * Alternatively, use appropriate barrier or GPU_finish before reading.