glsl: Remove const decoration from inlined function parameters

glsl: Remove const decoration from inlined function parameters

The constness of the function parameter gets inlined with the rest of
the function.  However, there is also an assignment to the parameter.
If this occurs inside a loop the loop analysis code will get confused
by the assignment to a read-only variable.

Fixes bugzilla #30552.

NOTE: this is a candidate for the 7.9 branch.