tmp fix for alpha by inverting mask of default:

[Shreeyak]

When locations is enabled, the alpha map is all white, because the background is geometry (not hdri).

• Alpha map is obtained by inverting the "default" segment in the cryptomatte manifest (which includes all the background geometry, for now).
• changed the masks for each obj to be floating point. added method to extract alpha

Warning: lots of methods tried, code complex. Needs a refactor.

Refer to: https://github.com/Synthesis-AI-Dev/convert_exr2png/issues/49