[GDNative] updates to string API and const fixes

[karroffel]

Hopefully the last time I change the API struct before the release.

cc @sheepandshepherd, @endragor, @touilleMan and also @BastiaanOlij, those should be affected by the changes the most.

[akien-mga]

Note to binding authors: we're likely going to release a RC1 build tomorrow or on Saturday, in case you want to sync your bindings with this breaking change before that.

[endragor]

@karroffel Given a UTF-8 string, how do you construct godot_string with this new API?

godot_string_new_data was removed, even though it's preferable (performance-wise and convenience-wise) to the two-step conversion: first to godot_char_string (performing allocations and copying along the way), then to godot_string. And even char * -> godot_char_string -> godot_string doesn't seem to be possible with this API, unless I missed it.

[karroffel]

@endragor,

godot_string str;
api->godot_string_new(&str);
api->godor_string_parse_utf8(&str, some_text);

[endragor]

@karroffel got it, thanks!

[BastiaanOlij]

So far I don't like this change... makes it far harder to do something that should be simple especially has GDNative will do a lot of interfacing with 3rd party apis which all have simple char buffers for strings and we'll be doing this alot.

The parse utf8 works around things, there should be a godot_string_parse_ascii then as well.

What I can't see is how to obtain a char * to the data held within a godot string.

[karroffel]

@BastiaanOlij every ASCII text is also valid UTF-8, so there is no need for a parse_ascii function.

To get a char pointer you do this

godot_char_string buf = api->godot_string_utf8(&str);

char *t = api->godot_char_string_get_data(&buf);
int len = api->godot_char_string_length(&buf);

// copy the data or use it otherwise

// free the Godot-allocated buffer
api->godot_char_string_destroy(&buf);

[BastiaanOlij]

Ah.. by ascii you mean text purely using the first 7 bits? the last 128 characters can't be used directly.
I figured it out in the end and got everything working again but I still feel its too many steps for something so often used :)

but hey, it works..