Fix basic suite narrowing conversions

[ellnor01]

Enabling narrowing errors by removing
if(CMAKE_COMPILER_IS_GNUCC OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "(Apple)?Clang") add_cxx_flag_if_supported(-Wno-narrowing) endif()
in test_conformance/basic/CMakeLists.txt produces many errors. A common example is:
test_conformance/basic/test_readimage_int16.cpp:87:49: error: narrowing conversion of 'img_width' from 'int' to 'size_t {aka long unsigned int}' inside { } [-Werror=narrowing] size_t region[3] = {img_width, img_height, 1};

[ellnor01]

This is a follow up for #787 and so requires #1144 to be merged first