Fix PCSS with WebGL1

[GSterbrant]

PCSS wouldn't compile with WebGL1 due to fetching a uniform value through a function. Moving the uniform value fetch to directly in the loop fixed the issue.

Fixes #5397

[slimbuck]

Any idea what the rule is here being broken in WebGL1?

[GSterbrant]

Using an argument to sample from a uniform makes the compiler complain that the value must be constant. What they mean is probably known at compile time.

It is also known at compile time obviously but the compiler probably sees the function outside of the scope of the callee and assumes it can be called anywhere, making it assume the uniform fetch can't be guaranteed to be invariant, which is what provokes the error.

But the error is simply Index expression must be constant.