glsl/glcpp: Don't use start-condition stack when switching to/from <DEFINE>

glsl/glcpp: Don't use start-condition stack when switching to/from <DEFINE>

This commit does not cause any behavioral change for any valid program. Prior
to entering the <DEFINE> start condition, the only valid start condition is
<INITIAL>, so whether pushing/popping <DEFINE> onto the stack or explicit
returning to <INITIAL> is equivalent.

The reason for this change is that we are planning to soon add a start
condition for <HASH> with the following semantics:

	<HASH>: We just saw a directive-introducing '#'

	<DEFINE>: We just saw "#define" starting a directive

With these two start conditions in place, the only correct behavior is to
leave <DEFINE> by returning to <INITIAL>. But the old push/pop code would have
returned to the <HASH> start condition which would then cause an error when
the next directive-introducing '#' would be encountered.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>