Bug 244008

Leak of id<MTLArgumentEncoder> in rx::(anonymous)::InitArgumentBufferEncoder().

The struct is defined like this in ProgramMtl.h:

```
struct ProgramArgumentBufferEncoderMtl
{
    void reset(ContextMtl *contextMtl);

    mtl::AutoObjCPtr<id<MTLArgumentEncoder>> metalArgBufferEncoder;
    mtl::BufferPool bufferPool;
};
```

However the method doesn't adopt the object in ProgramMtl.mm, which causes it to be assigned with a +2 retain count:

```
void InitArgumentBufferEncoder(mtl::Context *context,
                               id<MTLFunction> function,
                               uint32_t bufferIndex,
                               ProgramArgumentBufferEncoderMtl *encoder)
{
    encoder->metalArgBufferEncoder = [function newArgumentEncoderWithBufferIndex:bufferIndex];   // LEAK!
    if (encoder->metalArgBufferEncoder)
    {
        encoder->bufferPool.initialize(context, encoder->metalArgBufferEncoder.get().encodedLength,
                                       mtl::kArgumentBufferOffsetAlignment, 0);
    }
}
```