glthread/gallium: require safe_glthread to start glthread

glthread/gallium: require safe_glthread to start glthread

Print an error message for the user if the requirement isn't met, or
we're not thread safe.

v2: based on Nicolai feedbacks
Check the DRI extension version

v3: based on Emil feedbacks
improve commit and error messages.
use backgroundCallable variable to improve readability

v5: based on Emil feedbacks
Properly check the function pointer

Signed-off-by: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>