Merge topic 'expose_shader_api_release' into release

307a1345 Invoke UpdateShaderEvent from volume mapper
026ada0c Added test that demonstrates ability to set shader uniforms from python
41d80c54 Provide access to the shader program in UpdateShaderEvent callback

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Ben Boeckel <ben.boeckel@kitware.com>
Merge-request: !3613