Mouse: send wheel messages to hovered windows
VolumeCtrl: make mouse wheel consistent

Wheel up - volume up, wheel down - volume down.
And use the same steps as volume control command.
PlayerListCtrl: experimental internal timer hack
Controls: don't hide panels with mouse capture
Mouse: better double-click processing
Mouse: move popup menu notification to MpcEvent
VideoRenderers: blit correctly to negative windows

Needed for showing/auto-hiding docked panels and main menu without
rescaling the video
XySubPic: clip before memcpy