[Bf-funboard] Constant interpolation masks

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Using the mask tool the only interpolation mode is linear no matter what 
you set it to in the dope sheet.

It would be very useful if we could have masks at constant interpolation 
so the shape stays until the next key frame.

My user case right now is that I create a karaoke text effect in the VSE 
where I have an image containing the texts. I want them to appear one 
word at a time so I animate a mask modifier revealing the words. It is 
however in linear mode so in the frame before each frame  I have to 
duplicate the previous key. This is cumbersome, specially when I have to 
edit the mask. Constant interpolation would help a lot.

-d.