Convert RGB Table to use vtkTextureObject

Change-Id: Ibd2f72c816416461a466c202814545ef8afd02f4
ENH: Ability to load create alpha textures from raw data

Change-Id: I66ad0dfe04854ef9f9e1b458c818c75cb4d60377
Minor semantic changes

Change-Id: I1f827fbb385f129e65e0a09fccb124d23ca17518
ENH: Use texture object for opacity tables

Change-Id: Idfd54b44ca9d3075b94dddd935c5f3d9bbfaa088
ENH: Use vtkTextureObject for gradient opacity table

Change-Id: I274ac9c4a1f3e8088154638c902b9b35967932ed
ENH: Make sure the new API for opacity tables is supported by mapper

Change-Id: I2c8b37b8597b673d752ab8ac8844e2c39df2b935