lavc/dxv: fix incorrect back-reference index calculation in DXT5 decoding

This bug causes the DXT5 decoder to produce incorrect block texture data.
After the fix, textures are visually correct and match data decoded by
Resolume Alley (extracted with Nvida Nsight for comparison). Current FATE DXT5
samples did not cover this case.

Signed-off-by: Connor Worley <connorbworley@gmail.com>

libavcodec/dxv.c

@@ -955,7 +955,7 @@ static int dxv_decompress_dxt5(AVCodecContext *avctx)
                 break;
             case 2:
                 /* Copy two dwords from a previous index */
-                idx = 8 + bytestream2_get_le16(gbc);
+                idx = 8 + 4 * bytestream2_get_le16(gbc);
                 if (idx > pos || (unsigned int)(pos - idx) + 2 > ctx->tex_size / 4)
                     return AVERROR_INVALIDDATA;
                 prev = AV_RL32(ctx->tex_data + 4 * (pos - idx));