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.