ID 252771 - Channel Masks no longer evaluate the layer used as a mask correctly

Problem summary
When painting on a layer with a channel mask active, the channel mask acts as a solid layer and does not act as expected.

Customer reported version
3.2v1
Customer reported platform
centos7

Steps to reproduce

1) Open Mari 3.2v1 and create a project

2) Create a channel e.g. 'diffuse'.

3) Create a new Procedural Clouds layer. 

4) Right click on the Procedural Clouds layer and Convert to Paintable.

5) Navigate to the 'Projections' palette via (View > Palettes > Projection)

6) Toggle the Channel Mask menu within the Projections palette and select the 'diffuse' channel as the 'Mask Channel'.

7) Switch to a new channel, e.g. 'diffuse_2', and try to paint whilst the Channel Mask is active.

Workaround
Unknown. 

Reproduced by support
3.2v1
Windows 7, Centos 6, OSX Sierra

Regression
Last working version: 3.1v3

Expected behaviour
Channel Mask should mask against the Paint Buffer and evaluate the channel being used as a mask.

Actual behaviour
Acts as a solid 'white' mask meaning that painting layers does not work as expected.