vpxdec: Fix direct rendering, avoid holding write access

vpxdec: Fix direct rendering, avoid holding write access

When a buffer is pushed downstream, we should try not to hold the
buffer mapped with write access. Doing so would often lead to
an unneccesary memcpy later.

For instance, gst_buffer_make_writable() in
gst_video_decoder_finish_frame() will cause a memcpy because of
_memory_get_exclusive_reference().

We know that we can perform a two-step remap when using system
memory, as this will not cause the location of the memory to
change.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/812>