vpx_alpha: use picture context to forward plane

p_sys is used by the pool to forward the picture pool context. Pictures
that are supporting copying should be using picture context instead,
which the vpx_alpha decoder was already doing.

This fixes a huge memory leak of picture data when playing VP9 videos.

    STACK OF 1242 INSTANCES OF 'ROOT LEAK: malloc[2097152]':
    9   libsystem_pthread.dylib               0x185e89d34 thread_start + 8
    8   libsystem_pthread.dylib               0x185e8ef94 _pthread_start + 136
    7   libvlccore.9.dylib                    0x1032606cc DecoderThread + 348  decoder.c:1869
    6   libvlccore.9.dylib                    0x103262c04 DecoderThread_DecodeBlock + 256  decoder.c:1627
    5   ???                                   0x107066034 0x7fffffffffffffff + 9223372041267601461
    4   ???                                   0x12a809674 0x7fffffffffffffff + 9223372041862813301
    3   ???                                   0x107065c70 0x7fffffffffffffff + 9223372041267600497
    2   ???                                   0x107066644 0x7fffffffffffffff + 9223372041267603013
    1   ???                                   0x1070662ec 0x7fffffffffffffff + 9223372041267602157
    0   libsystem_malloc.dylib                0x185cc8a68 _malloc_zone_malloc_instrumented_or_legacy + 148 

Instruments correctly indicates the following trace:

    CombinePicturesCPU	
    SendMergedLocked	
    QueuePic	
    ecoder_QueueVideo	
    ecodeBlock	
    DecodeVideo	
    Decode	
    DecoderThread_DecodeBlock	
    DecoderThread	
    _pthread_start	
    thread_start

See ticket for details of the trace.

Fixes #28792