GfXShading: Prepend some variables with bbox_

Makes it clear those variables are only about the bbox, also fixes shadow warnings
6 jobs for fix_more_shadows in 8 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #374837
build

00:03:36

passed #374842
build_android

00:01:52

passed #374838
build_clang_libcpp

00:03:34

passed #374841
build_clazy

00:08:34

passed #374840
build_mingw64_fedora

00:03:21

passed #374839
build_ubuntu_16_04

00:01:59