[glsl-in] Allow dynamic indexing on constant variables

Previously we always set the lhs flag when lowering to generate a
pointer so that dynamic indexing would work, this would produce an error
on constant variables since they can't be in a lhs context.

Now we introduce an enum which distinguishes not only between lhs and
rhs but also in array base, if lowering in a lhs context the base is
also lowered in a lhs context but if lowering in rhs the base is lowered
in a special array base context which bypasses the mutability check.

Fixes #1237