PSOutputDev/pdftops: for splashModeCMYK8 and language level >=2 activate overprint emulation

The overprint operands are always emitted for any language level >= 2, such that it is just consistent
to activate overprint emulation during rasterization if it is quite likely that the final device
supports this operation.
Status Job ID Name Coverage
  Build
passed #2989117
build

00:03:23

passed #2989122
build_android

00:00:47

passed #2989118
build_clang_libcpp

00:03:31

passed #2989121
build_clazy

00:02:48

passed #2989120
build_mingw64_fedora

00:02:00

passed #2989119
build_ubuntu_16_04

00:01:59