direct2d: do not always create bitmap

have not compared the performance
TODO:
1. how to update bitmap when setSource called and size changes
2. change base class from ImageRenderer to VideoRenderer to avoid QImage
constructing
code style, var names etc.
direct2d: inherit VideoRenderer to avoid QImage construct

TODO:
1. need a function call when setSourceSize() called
2. try QPainter::beginNativePaint()
3. try to implement other QPaintEngine. What Qt has?
direct2d: check ptr

BUG:
sometimes windows give this error: "D3D11: Removing Device"
even the user will logout the system!
direct2d: showEvent instead of changeEvent to avoid flicker when active
state changed
gdi: showEvent instead of changeEvent
vo template: use showEvent, inherits VideoRenderer
refine code etc
Merge branch 'directx'
remove currentFrameImage() because of other paint engine