Pass length constrained mrb string to osc-bridge

When views are created on mruby strings it's possible that the string
pointer points to a longer string than is visible at the ruby level.
In other words strlen(s.pointer) >= s.length. By copying a string we
avoid this edge case.

In the UI this created a bug where it was impossible to delete
characters from a long string.
In ruby you'd start with a string like "foobar"\0(len 6), delete a
character off the end via creating a string view which at the ruby
level was "fooba", but under the hood it was represented via
"fooba"r\0(len 5).
This specific behavior only occurs for non-embeddable strings which in
mruby's case is about roughly 27 characters (compilation flag
dependent).

src/mruby-widget-lib/src/gem.c

@@ -1134,6 +1134,20 @@ mrb_remote_param_set_value_ar(mrb_state *mrb, mrb_value self)
     return self;
 }
 
+static char *
+mrb_copy_str(mrb_state *mrb, mrb_value value)
+{
+    size_t      l   = mrb_string_value_len(mrb, value);
+    const char *s   = mrb_string_value_ptr(mrb, value);
+    char       *out = malloc(l+1);
+
+    for(int i=0; i<l; ++i)
+        out[i] = s[i];
+
+    out[l] = 0;
+    return out;
+}
+
 static mrb_value
 mrb_remote_param_set_value_str(mrb_state *mrb, mrb_value self)
 {
@@ -1148,8 +1162,9 @@ mrb_remote_param_set_value_str(mrb_state *mrb, mrb_value self)
     mrb_assert(param->br);
     mrb_assert(param->uri);
 
-    const char *str = mrb_string_value_ptr(mrb, value);
+    const char *str = mrb_copy_str(mrb, value);
     br_set_value_string(param->br, param->uri, str);
+    free(str);
     return self;
 }