* Correct khronos extension device version checks so that 2.0, 2.1 and
2.2 extensions are only require for devices versions < 3.0
(`cl_khr_3d_image_writes`, `cl_khr_image2d_from_buffer` and
`cl_khr_depth_images` are all optional for devices supporting
OpenCL-3.0).
* Remove `cl_khr_fp64` from list or required extensions for 2.0,
`cl_khr_fp64` is a required extension for OpenCL-1.2 devices if they
suport doubles. Check for support of `cl_khr_fp64` on 1.2 devices.
* Remove duplicated `cl_khr_byte_addressable_store` from list of 2.0
extensions. Since this is a 1.1 extension and all versions greater than
1.1 are backwards compatible with 1.1 this will be checked anyway.
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