Merge topic '2d-transfer-func'

48e2a4c1 Simplified independent/dependent component mode selection for 1D_TF.
05af3d3f Added test for transfer function 2D.
4e157028 Added fragment shader code for 2D Transfer Functions.
ac38b6a0 Using opacityTablesMap for consistency.
11c36123 Implemented TransferFunction2D initialization and clean-up.
5883a1a3 Fixed graphics mem leak when re-initializing transfer 1D.
23abf8b9 Adding support for 2d-transfers in volume mapper.
fc57b9fc Added 2D transfer functions to vtkVolumeProperty's API.
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Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Marcus D. Hanwell <marcus.hanwell@kitware.com>
Merge-request: !2809