lavc/texturedsp: fix premult2straight inversion

This function should convert premultiplied alpha to straight, but does the opposite.

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

libavcodec/texturedsp.c

@@ -175,9 +175,12 @@ static av_always_inline void premult2straight(uint8_t *src)
     int b = src[2];
     int a = src[3]; /* unchanged */
 
-    src[0] = (uint8_t) r * a / 255;
-    src[1] = (uint8_t) g * a / 255;
-    src[2] = (uint8_t) b * a / 255;
+    if (a == 0)
+        return;
+
+    src[0] = (uint8_t) FFMIN(r * 255 / a, 255);
+    src[1] = (uint8_t) FFMIN(g * 255 / a, 255);
+    src[2] = (uint8_t) FFMIN(b * 255 / a, 255);
 }
 
 /**

tests/ref/fate/dds-dxt2

@@ -3,4 +3,4 @@
 #codec_id 0: rawvideo
 #dimensions 0: 64x64
 #sar 0: 0/1
-0,          0,          0,        1,    16384, 0x11cebeb0
+0,          0,          0,        1,    16384, 0xd7f7241b

tests/ref/fate/dds-dxt4

@@ -3,4 +3,4 @@
 #codec_id 0: rawvideo
 #dimensions 0: 64x64
 #sar 0: 0/1
-0,          0,          0,        1,    16384, 0x31aaacd6
+0,          0,          0,        1,    16384, 0xf18d4216