x11: ensure some context is bound before calling glClientWaitSync

x11: ensure some context is bound before calling glClientWaitSync

Since commit 972134abe48a4c9c7b6ad41b0723f30f4e7ae16b we now call
glClientWaitSync for the vendor nvidia driver, to know when a frame
is ready for the compositor to process.

glClientWaitSync can be called regardless of which context is currently
bound, but if no context is bound at all, it returns 0 without
doing anything.

This commit checks for that edge case, and ensures a context gets
made current in the event no context is already current, before calling
glClientWaitSync.