cvpx: add smarter CVPX->SW chroma+resize converter

CVPX -> CVPX chroma conversion filter is using HW-accelerated chroma
conversion (VTPixelTransfer) on MacOSX, which is also able to resize
pictures efficiently.

However, when generating pictures with VLC_CODEC_CVPX_BGRA as i_chroma,
the core will add the following conversion chain:

    CVPB (WxH) -> BGRA (WxH) -> I422 (WxH) -> I420 (W'xH')

Instead, by using this new filter with a higher priority, we can detect
that resize must be performed between CVPX buffers and directly convert
to the most suitable CVPX chroma type, leading to the following
conversion chain:

    CVPB (WxH) -> CVPI (W'xH') -> I420 (W'xH')

Which has the benefit to never use swscale while avoiding an additional
conversion, and transform the last conversion into only a memory
mapping.