glsl: Initialize all member variables of _mesa_glsl_parse_state on construction.

glsl: Initialize all member variables of _mesa_glsl_parse_state on construction.

The _mesa_glsl_parse_state object relies on the memory allocator
zeroing out its contents before it's initialized, which is quite an
unusual practice in the C++ world because it ties objects to some
specific allocation scheme, and gives unpredictable results when an
object is created with a different allocator -- Stack allocation,
array allocation, or aggregation inside a different object are some of
the useful possibilities that come to my mind.  Initialize all fields
from the constructor and stop using the zeroing allocator.

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>