/usr/share/gazebo-9/media/rtshaderlib/ is in gazebo9-common 9.0.0+dfsg5-3ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/share/gazebo-9/media/rtshaderlib/FFPLib_Common.glsl
/usr/share/gazebo-9/media/rtshaderlib/FFPLib_Fog.glsl
/usr/share/gazebo-9/media/rtshaderlib/FFPLib_Lighting.glsl
/usr/share/gazebo-9/media/rtshaderlib/FFPLib_Texturing.glsl
/usr/share/gazebo-9/media/rtshaderlib/FFPLib_Transform.glsl
/usr/share/gazebo-9/media/rtshaderlib/SGXLib_IntegratedPSSM.glsl
/usr/share/gazebo-9/media/rtshaderlib/SGXLib_NormalMapLighting.glsl
/usr/share/gazebo-9/media/rtshaderlib/SGXLib_PerPixelLighting.glsl
/usr/share/gazebo-9/media/rtshaderlib/SampleLib_ReflectionMap.glsl
/usr/share/gazebo-9/media/rtshaderlib/materials/
/usr/share/gazebo-9/media/rtshaderlib/materials/RTShaderSystem.material