VBufBackend_t::cancelPendingUpdate: reset renderThreadTimerID to 0 after killing...

VBufBackend_t::cancelPendingUpdate: reset renderThreadTimerID to 0 after killing the timer. This will stop any possible pending updates from happening that were still possibly in the windows message queue after KillTimer was called. This more importantly also fixes  a problem introduced by r4356 where after a text change and focus change happen in quick succession only focus changes will update the buffer after that point. E.g. after changing the value in a combo box, edit fields would never update their content again.