This patch doesn't implement support for 3-channel formats,
it just allows it in the spec and implementation
so that a backend could implement it later (e.g. Level Zero).

* Removed check from `image_descriptor::verify`
* Updated spec to allow 3-channel formats
* Very simple test, just verifies that CUDA backend still fails
  * Missing some PTX intrinsics, use `float4` instead of `float4`
    when compiling for CUDA. The more important part is checking that
    the CUDA backend throws an exception when trying to create image.

Co-authored-by: Wenju He <wenju.he@intel.com>

Currently no way to query whether a backends supports 3 channel formats.

# Conflicts:
#	sycl/doc/extensions/experimental/sycl_ext_oneapi_bindless_images.asciidoc

# Conflicts:
#	sycl/doc/extensions/experimental/sycl_ext_oneapi_bindless_images.asciidoc