Image conversion rules duplicated between environment spec and OpenCL C spec

See https://github.com/KhronosGroup/OpenCL-Docs/pull/1352 for an example of an extension that needs to add the same (lenghty) descriptions twice.

Start by reviewing the example in pull request 1352 and compare where the image conversion descriptions appear in the environment and OpenCL C specifications. Determine what shared source or generation entry points are available, then verify that an extension can include the rules without duplicating the lengthy text.

Written by the indexing model from the issue text.