glsl: Have the lexer return LAYOUT_TOK if 420pack is enabled.
glsl: Have the lexer return LAYOUT_TOK if 420pack is enabled.

GL_ARB_shading_language_420pack also provides layout qualifiers.

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Paul Berry <de21b9f22e6dd0bdf7890875d6e07ef4967c44ce@gmail.com>

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