vdpau: fix importing surface from main memory
On the one hand, when decoding in hardware, the video surface dimensions will match the coded dimensions, rather than the visible ones. Otherwise the hardware decoder would not have enough space for its output. In this the libavcodec plugin does not add padding (since libavcodec SIMD optimizations are not involved). On the other hand, when decoding in software, the video surface height must not include the padding added by libavcodec SIMD optimizations. So then the visible height needs to be used to avoid bleeding green lines.
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