mpegvideo: increase edge_emu_buffer size for VC1
The VC1 decoder uses edge_emu_buffer simultaneously for luma and chroma and needs more space. That was not a problem before f1d8763a since the size for edge_emu_buffer was always calculated with 2 byte per pixel since the linesize was not known. Fixes occasionally fate errors in vc1_sa10143.
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