PSOutputDev: Don't read outside of image buffer

Check whether input image is RGB or BGR to not treat
it as CMYK in those cases in PSOutputDev::checkPageSlice().

Fixes #751
6 jobs for PSOutputDev-Dont-read-outside-of-image-buffer in 4 minutes and 4 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #435870
build

00:03:02

passed #435875
build_android

00:00:59

passed #435871
build_clang_libcpp

00:03:16

passed #435874
build_clazy

00:04:03

passed #435873
build_mingw64_fedora

00:02:55

passed #435872
build_ubuntu_16_04

00:01:41