Fix misdetection of GLX_EXT_buffer_age
We were detecting GLX_EXT_buffer_age
as supported even when it's not. Because it appeared in the list of client-supported extensions, but not the list of server-supported extensions.
Closes: #3007 (closed)
although I would prefer to first understand why mtk_x11_error_trap_push
is failing to do its job which led us here.