glx: Do not call into Composite if it is disabled.

Otherwise X server crashes if GLX is enabled and Composite disabled. For example the compIsAlternateVisual function will try to lookup CompScreenPtr using the CompScreenPrivateKey, but that was never initialized if Composite is disabled.

Fixes: f84e59a4. ("glx: Duplicate relevant fbconfigs for compositing visuals") Bugzilla: Signed-off-by: Michal Srb

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