glsl: Make vertex ID lowering declare gl_BaseVertex as hidden.

glsl: Make vertex ID lowering declare gl_BaseVertex as hidden.

If the GL_ARB_shader_draw_parameters extension is enabled, we'll already
have a gl_BaseVertex variable.  It will have var->how_declared set to
ir_var_declared_implicitly, and will appear in the program resource
list.

If not, we make one for internal use.  We don't want it to be listed
in the program resource list, as the application won't be expecting
it.  Marking it hidden will properly exclude it.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>