lavc/dxv: assume DXV2 files use premultiplied alpha

I generated a DXV2 file with an interesting alpha channel using
Adobe Media Encoder 2015 and compared decoding it using Resolume Alley
and ffmpeg. Similarly to DXV3 files, Alley expects premultiplied alpha
and ffmpeg matches its decoding more closely when it does the same.

Reference file: https://connorworley.com/dxv2-dxt5.mov

Existing FATE tests for DXV2 files do not cover this change.
Signed-off-by: Connor Worley <connorbworley@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/dxv.c

@@ -919,7 +919,7 @@ static int dxv_decode(AVCodecContext *avctx, AVFrame *frame,
             tag = DXV_FMT_DXT5;
             msgtext = "DXT5";
 
-            texdsp_ctx.tex_funct = ctx->texdsp.dxt5_block;
+            texdsp_ctx.tex_funct = ctx->texdsp.dxt4_block;
             texdsp_ctx.tex_ratio = 16;
             texdsp_ctx.raw_ratio = 16;
         } else if (old_type & 0x20 || version_major == 1) {