Glow node changed behavior in 2.4.1 because of #647

[devernay]

This issue cause the following unit tests to fail in 2.4.1:

• TestGlow
• BayMax (uses Glow)

The reason is that Glow uses a Merge(multiply) which multiplies with the optional mask (highlighted):

Problem:

• the previous behavior of Multiply was to return input B if A (here, the mask) was not connected.
• since #647 this defaults to black and transparent if A is not connected

The behavior introduced in #647 is the right one, and is consistent with Nuke.

How do we fix Glow so that it defaults to a solid mask if it's not connected? We probably need to insert another node here.

[devernay]

Solution:

1. set an expression on the "disable" parameter of the Merge(multiply) node above: "thisGroup.getInput(1) is None": this disables the node if the mask input is not connected. Implemented in https://github.com/NatronGitHub/Natron/commit/3ac7b02f305241d6802e387bdc6b281e96b9992c - this is also necessary on Nuke, see https://benmcewan.com/blog/2019/10/21/back-to-basics-common-errors-when-creating-a-gizmo/
2. increment the Merge node version number so that old projects still work. Implemented in https://github.com/NatronGitHub/openfx-misc/commit/82f5f5d4100c6735f3efbe957b768cd085701ed5