Cycles: Fix animation hangs/crashes in Metal due to leaking temp objects

This PR adds `@autoreleasepool` blocks around functions that have been observed to create hidden temporary NSObjects, and eventually cause command buffer failures. A couple of allocations needed to be tweaked in order to maintain correct retain/release behaviour. This PR also fixes the command buffer error text to show more useful information.

intern/cycles/device/metal/device_impl.h

@@ -26,6 +26,7 @@ class MetalDevice : public Device {
   id<MTLLibrary> mtlLibrary[PSO_NUM] = {nil};
   id<MTLArgumentEncoder> mtlBufferKernelParamsEncoder =
       nil; /* encoder used for fetching device pointers from MTLBuffers */
+  id<MTLCommandQueue> mtlComputeCommandQueue = nil;
   id<MTLCommandQueue> mtlGeneralCommandQueue = nil;
   id<MTLArgumentEncoder> mtlAncillaryArgEncoder =
       nil; /* encoder used for fetching device pointers from MTLBuffers */