poppler: Correct bitmap memory handling on output

Use dataAt, alphaAt, and casts in SplashOutputDev.
7 jobs for patchset in 5 minutes and 54 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #4826771
build

00:05:54

passed #4826776
build_android

00:00:42

passed #4826772
build_clang_libcpp

00:05:40

passed #4826775
build_clazy

00:03:57

passed #4826774
build_mingw64_fedora

00:02:54

passed #4826773
build_ubuntu_16_04

00:02:48

passed #4826770
clang_format

00:01:12