glsl: Use has_layout() rather than a partial open coded version.

glsl: Use has_layout() rather than a partial open coded version.

The idea of this code is to disallow layout(...) sections with the
deprecated "varying" or "attribute" keywords, unless a few select
extensions are enabled which allow a more relaxed check.

In order to detect a layout(...) section, the code checks for a number
of layout qualifiers.  However, it failed to check for all of them,
which could lead to layout(...) not being detected when it should.

By replacing this with has_layout(), we properly check for all layout
qualifiers, and also guarantees that new qualifiers added in the future
will not be forgotten.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>