This change makes keyboard inputs more responsive on Linux, especially
when the FPS is lower on slower configurations.
Polling events from the x server is done on a separate thread to avoid a
frame delay with inputs, due to first sending the event to the input
manager with XFilterEvent then processing the new event only on the next
frame.
Calls to Input Manager functions like XSetICFocus, XUnsetICFocus and
XSetICValues use a mutex, because they are polling events internally and
would otherwise interfere with our own thread process for polling events
which can cause a deadlock in some cases.
XUnsetICFocus is called instead of XSetICFocus on FocusOut events,
so the input manager can be properly notified of focus changes.
clipboard_get now uses a blocking call to poll for a specific event type
when waiting for a SelectionNotify event, instead of polling all events
and filtering them afterwards.
2c4d64102a
