MaskPass: Add stencil state locking in MaskPass effect

[gkjohnson]

See #15611 (comment)

This change adds stencil locking to the MaskPass effect code and modifies to stencil state to respect the lock state when enabling / disabling the stencil buffer. In my opinion the setOp and setFunc functions should also respect the locked state but I'll wait for feedback before making that change, too.

Note that this PR requires a new build before the webgl_postprocessing_masking example will work again.

[WestLangley]

@mrdoob After testing, I believe this is an acceptable fix for r.107.

[mrdoob]

Doesn't seem like this deserves a hotfix for r107. Merging for 108.

[mrdoob]

Thanks!