Add example or test case for `writeToFn`

[loftafi]

Thank you for putting this package together to make it easier for zig users to use this library.

zstbi.Image.writeToFn(img, write_fn: *const fn (?*anyopaque, ?*anyopaque, c_int) void, context: ?*anyopaque, format);

It is not clear to a beginner to this library, how to use the writeToFn function. How are the parameters of the function supposed to be used? I looked in the zig file and there is no test for using it that might give a clue.

It would be helpful if a more advanced user of this library could add an example to the README.md or a test case in the zig code.

Thanks again for putting this project together.

---

Just guessing, I presume we need to do something like this:

const format: zstbi.ImageWriteFormat = .{ .jpg = .{ .quality = 75 } };
var buffer: Buffer = .{ .allocator = allocator };
try zstbi.Image.writeToFn(img, write_fn, &buffer, format);

const Buffer = struct {
    allocator: Allocator,
    data: std.ArrayListUnmanaged(u8) = .empty,
};

fn write_fn(context: ?*anyopaque, data: ?anyopaque, size: c_int) void {
    var buffer: *Buffer = @ptrCast(context);
    // Append data to buffer, maybe something like this???
    buffer.data.append(buffer.allocator, data[0..size]);
}

[loftafi]

For anyone else stumbling across this, here is a functional/working example:

var buffer: Buffer = .{ .allocator = allocator };
try zstbi.Image.writeToFn(img, write_fn, &buffer, .png);

const Buffer = struct {
    allocator: Allocator,
    data: std.ArrayListUnmanaged(u8) = .empty,
    failed: bool = false,
};

fn write_fn(context: ?*anyopaque, data: ?*anyopaque, size: c_int) callconv(.c) void {
    var buffer: *Buffer = @ptrCast(@alignCast(context));
    var block: [*]const u8 = @ptrCast(data);
    if (buffer.failed) return;
    buffer.data.appendSlice(buffer.allocator, block[0..@as(usize, @intCast(size))]) catch |f| {
        err("Failed appending image data. {any}", .{f});
        buffer.failed = true;
    };
}