mesa: rework the way we manage extra_extensions

mesa: rework the way we manage extra_extensions

Store pointers to the tokenized strings in the gl_extensions struct.

This way we can reuse them in glGetStringi() while we construct the
really long string only in _mesa_make_extension_string.

Only 16 pointers/strings are stored for now.

v2: Warn only once when we provide more than 16 unk. extensions, rebase

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Brian Paul <brianp@vmware.com> (v1)