api: create QR codes from any data (#6090)

this PR adds a function that can be used to create any QR code, in a raw
form.

this can be used to create add-contact as well as add-second-device QR
codes (eg. `dc_create_qr_svg(dc_get_securejoin_qr())`) - as well as for
other QR codes as proxies.

the disadvantage of the rich-formatted QR codes as created by
`dc_get_securejoin_qr_svg()` and `dc_backup_provider_get_qr_svg()` were:

- they do not look good and cannot interact with UI layout wise (but
also tapping eg. an address is not easily possible)
- esp. text really looks bad. even with
[some](e5dc8fe3d8)
[hacks](https://github.com/deltachat/deltachat-android/pull/2215) it
[stays buggy](https://github.com/deltachat/deltachat-ios/issues/2200);
the bugs mainly come from different SVG implementation, all need their
own quirks
- accessibility is probably bad as well

we thought that time, SVG is a great thing for QR codes, but apart from
basic geometrics, it is not.

so, we avoid text, this also means to avoid putting an avatar in the
middle of the QR code (we can put some generic symbol there, eg.
different ones for add-contact and add-second-device).

while this looks like a degradation, also other messengers use more raw
QR codes. also, we removed many data from the QR code anyway, eg. the
email address is no longer there. that time, sharing QR images was more
a thing, meanwhile we have invite links, that are much better for that
purpose.

in theory, we could also leave the SVG path completely and go for PNG -
which we did not that time as PNG and text looks bad, as the system font
is not easily usable :) but going for PNG would add further challenges
as passing binary data around, and also UI-implemtation-wise, that would
be a larger step. so, let's stay with SVG in a form we know is
compatible.

the old QR code functions are deprecated.

deltachat-ffi/deltachat.h

@@ -2611,6 +2611,7 @@ char*           dc_get_securejoin_qr         (dc_context_t* context, uint32_t ch
  * Get QR code image from the QR code text generated by dc_get_securejoin_qr().
  * See dc_get_securejoin_qr() for details about the contained QR code.
  *
+ * @deprecated 2024-10 use dc_create_qr_svg(dc_get_securejoin_qr()) instead.
  * @memberof dc_context_t
  * @param context The context object.
  * @param chat_id group-chat-id for secure-join or 0 for setup-contact,
@@ -2791,6 +2792,22 @@ dc_array_t* dc_get_locations                (dc_context_t* context, uint32_t cha
 void        dc_delete_all_locations         (dc_context_t* context);
 
 
+// misc
+
+/**
+ * Create a QR code from any input data.
+ *
+ * The QR code is returned as a square SVG image.
+ *
+ * @memberof dc_context_t
+ * @param payload The content for the QR code.
+ * @return SVG image with the QR code.
+ *     On errors, an empty string is returned.
+ *     The returned string must be released using dc_str_unref() after usage.
+ */
+char*           dc_create_qr_svg             (const char* payload);
+
+
 /**
  * Get last error string.
  *
@@ -2879,6 +2896,7 @@ char* dc_backup_provider_get_qr (const dc_backup_provider_t* backup_provider);
  * This works like dc_backup_provider_qr() but returns the text of a rendered
  * SVG image containing the QR code.
  *
+ * @deprecated 2024-10 use dc_create_qr_svg(dc_backup_provider_get_qr()) instead.
  * @memberof dc_backup_provider_t
  * @param backup_provider The backup provider object as created by
  *    dc_backup_provider_new().
@@ -2918,7 +2936,7 @@ void dc_backup_provider_unref (dc_backup_provider_t* backup_provider);
  * Gets a backup offered by a dc_backup_provider_t object on another device.
  *
  * This function is called on a device that scanned the QR code offered by
- * dc_backup_sender_qr() or dc_backup_sender_qr_svg().  Typically this is a
+ * dc_backup_provider_get_qr().  Typically this is a
  * different device than that which provides the backup.
  *
  * This call will block while the backup is being transferred and only

deltachat-ffi/src/lib.rs

@@ -30,7 +30,7 @@ use deltachat::ephemeral::Timer as EphemeralTimer;
 use deltachat::imex::BackupProvider;
 use deltachat::key::preconfigure_keypair;
 use deltachat::message::MsgId;
-use deltachat::qr_code_generator::{generate_backup_qr, get_securejoin_qr_svg};
+use deltachat::qr_code_generator::{create_qr_svg, generate_backup_qr, get_securejoin_qr_svg};
 use deltachat::stock_str::StockMessage;
 use deltachat::webxdc::StatusUpdateSerial;
 use deltachat::*;
@@ -2594,6 +2594,18 @@ pub unsafe extern "C" fn dc_delete_all_locations(context: *mut dc_context_t) {
     });
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn dc_create_qr_svg(payload: *const libc::c_char) -> *mut libc::c_char {
+    if payload.is_null() {
+        eprintln!("ignoring careless call to dc_create_qr_svg()");
+        return "".strdup();
+    }
+
+    create_qr_svg(&to_string_lossy(payload))
+        .unwrap_or_else(|_| "".to_string())
+        .strdup()
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn dc_get_last_error(context: *mut dc_context_t) -> *mut libc::c_char {
     if context.is_null() {