ID 130232 - The bounding box of the mask input is included when outputting a Keymix node with the bbox set to union.

Problem summary
The mask input is included when outputting a Keymix node with a bbox set to union.

Customer reported version
Nuke 10.5v7

Customer reported platform
CentOS 7

Steps to reproduce
1) From Nuke, add a Keymix node to the Node graph.

2) Create and attach Roto nodes to the A, B, and mask inputs of the Keymix node.

3) Double-click the Keymix node to access the Properties bin.

4) Ensure the bbox option is set to Union.

5) Select and Disable the Roto node connected to the mask input of the Keymix node.

6) View the bounding box in the Viewer and re-enable the Roto node mask input.

Reproduced by support in:
Nuke 11.2v3 - Windows 10, CentOS 7, macOS 10.13 High Sierra
Nuke 11.2v1 - Windows 10
Nuke 11.1v6 - Windows 10
Nuke 11.1v1 - Windows 10
Nuke 11.0v4 - Windows 10
Nuke 11.0v1 - Windows 10
Nuke 10.5v7 - Windows 10
Nuke 10.5v1 - Windows 10
Nuke 10.0v6 - Windows 10
Nuke 10.0v1 - Windows 10
Nuke 9.0v9 - Windows 10
Nuke 9.0v1 - Windows 10
Nuke 8.0v7 - Windows 10
Nuke 8.0v1- Windows 10
Nuke 7.0v10 - Windows 10, CentOS 7, macOS 10.13 High Sierra

Earliest version tested:
Nuke 7.0v10 - This issue appears to be in all versions of the product

Earliest version tested:
Nuke 7.0v10 - Windows 10, CentOs 7, macOS 10.13 High Sierra

Expected behavior:
The bounding box should only be applied from the Union of the A and B inputs on the Keymix node.

Actual behavior:
The bounding box includes the mask input of the Keymix node.