WebGLProgram: Simplify condition.

[Mugen87]

Related issue: -

Description

Instead of having !! material.sheen > 0 it is sufficient to have material.sheen > 0 because the result of this statement is already a boolean.

[gkjohnson]

Just something I felt an urge to understand because the current behavior wasn't obvious to me but it seems the !! material.sheen term evaluates first so the current implementation actually tests true > 0 or false > 0. Maybe some good JS order of operations trivia knowledge

[mrdoob]

All that ! foo and !! foo stuff is evil...

[mrdoob]

Thanks!