[loplugin:staticmethods].

And then 'const' makes no sense.

Also, fix fallout warning: calling static member function through
member call syntax, use 'ColorMaskElement::CalcMaskShift' instead
[loplugin:staticcall].

But actually I wonder if this should be a non-static member function
that operates on 'this' (and takes no parameter) instead? Also, the
return value is unused.

Change-Id: I24949b74bd43eb90499e3c881941b102a3622f92