glsl: encapsulate binding validation and setting

glsl: encapsulate binding validation and setting

This change moves the binding layout handing code into an apply
function to be consistent with other helper functions in the ast
code, and to encapsulate the code so that when we introduce
compile time constants the code will be much cleaner.

One small downside is for unnamed interface blocks we will now
be revalidating the binding for each member its applied to.
However this seems a small sacrifice in order to have code which
is readable.

We also remove the incorrect comment in the named interface code
about propagating bindings to members which seems to have been
copied from the unnamed interface code.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>