swscale/ops: clarify SwsOpList.src/dst semantics

Turns out these are not, in fact, purely informative - but the optimizer
can take them into account. This should be documented properly.

I tried to think of a way to avoid needing this in the optimizer, but any
way I could think of would require shoving this to SwsReadWriteOp, which I
am particularly unwilling to do.

libswscale/ops.h

@@ -209,8 +209,8 @@ typedef struct SwsOpList {
     SwsOp *ops;
     int num_ops;
 
-    /* Purely informative metadata associated with this operation list */
-    SwsFormat src, dst;
+    /* Metadata associated with this operation list */
+    SwsFormat src, dst; /* if set; may inform the optimizer about e.g value ranges */
 } SwsOpList;
 
 SwsOpList *ff_sws_op_list_alloc(void);