differentiate frame size and surface/image size

the frame size in a VaapiSurface changes on the fly, refer to vp8dec code; that's why I don't keep a mirror/cache for frame size in VaapiImage.
VaapiImage::derive, it is possible that VaapiSurface's frame size gets update after ::derive.
Vaapi::create, for image created from Vaapi::create, it has no associate VaapiSurface; so it will reach "return getHeight();"