Merge topic 'volume_rendering_export_depth_texture2'

6694a0fa Fixed failing test
2a22829d Moved render to texture API to base class
9f7027e4 Removed debug code
c2cd9fb8 Use unsigned char data type for image data
603d923b Export alpha channel and single component depth texture
fb04a30b Merge branch 'master' into volume_rendering_export_depth_texture
ea3e57a6 Baselines for new tests
6b31b8bc Pass the output image by reference for the mapper to dump to
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Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !586