m_config: optimize initialization of each option

Options with dynamic memory allocations (such as strings) require some
care. They need to be fully copied on initialization, and if a static
default value was declared, we must not free that value either.

Instead of going through the entire thing even for simple types like
integers, really run it only for options with dynamic allocations. To
distinguish types which use dynamic allocations, we can use the fact
that they require a free callback (otherwise they would leak). As a
result initialization of simple types becomes chaper, and the init
function does nothing at all if src==dst for a simple type.

(It's funny how mplayer had M_OPT_TYPE_DYNAMIC since 2002, until we
replaced it by the same heuristic as used here in commit 3bb134969e.
It's also funny how the new check was used only for some asserts, and
finally removed in commit 7539928c1c. I guess at this time I felt like
having uniform code was more important than pointless
micro-optimizations.)

The src==NULL case is removed because it can't happen.

options/m_config.c

@@ -178,15 +178,19 @@ static void substruct_write_ptr(void *ptr, void *val)
 }
 
 // Initialize a field with a given value. In case this is dynamic data, it has
-// to be allocated and copied. src can alias dst, also can be NULL.
+// to be allocated and copied. src can alias dst.
 static void init_opt_inplace(const struct m_option *opt, void *dst,
                              const void *src)
 {
-    union m_option_value temp = {0};
-    if (src)
+    // The option will use dynamic memory allocation iff it has a free callback.
+    if (opt->type->free) {
+        union m_option_value temp;
         memcpy(&temp, src, opt->type->size);
-    memset(dst, 0, opt->type->size);
-    m_option_copy(opt, dst, &temp);
+        memset(dst, 0, opt->type->size);
+        m_option_copy(opt, dst, &temp);
+    } else if (src != dst) {
+        memcpy(dst, src, opt->type->size);
+    }
 }
 
 static void alloc_group(struct m_config_data *data, int group_index,