Context/images: add ccl_context_get_supported_image_formats()

- Add test (passing)
- Add OCL stub clGetSupportedImageFormats() implementation
- The implementation of ccl_context_get_supported_image_formats()
is awkard, because it returns a zero-terminated list of supported
image formats. This should be improved.
- ccl_context_new_from_devices_full() now accepts CCLDevice* const*
instead of CCLDevice**.