An event. This can be used to identify async copies from global
to local memory and vice-versa. See individual Async Copy and Prefetch
Functions for more information.

NOTE: The image2d_t, image3d_t, image2d_array_t,
image1d_t, image1d_buffer_t, image1d_array_t,
and sampler_t types are only defined if the device
supports images i.e. CL_DEVICE_IMAGE_SUPPORT
is CL_TRUE. See the table for param_name for
clGetDeviceInfo for details.