swscale/ops_chain: add ability to match fixed scale factor

This is useful especially for the special case of scaling by common
not-quite-power-of-two constants like 255 or 1023.

Signed-off-by: Niklas Haas <git@haasn.dev>

libswscale/ops_chain.c

@@ -184,7 +184,7 @@ static int op_match(const SwsOp *op, const SwsOpEntry *entry, const SwsComps nex
         score += av_popcount(SWS_MASK_ALL ^ entry->linear_mask);
         return score;
     case SWS_OP_SCALE:
-        return score;
+        return av_cmp_q(op->c.q, entry->scale) ? 0 : score;
     case SWS_OP_TYPE_NB:
         break;
     }

libswscale/ops_chain.h

@@ -111,6 +111,7 @@ typedef struct SwsOpEntry {
         uint32_t       linear_mask; /* subset of SwsLinearOp */
         int            dither_size; /* subset of SwsDitherOp */
         int            clear_value; /* clear value for integer clears */
+        AVRational     scale;       /* scale factor for SWS_OP_SCALE */
     };
 
     /* Kernel implementation */