rebaseGrowing

fn rebaseGrowing(mr: *MultiReader, context: *Context, capacity: usize) Allocator.Error!void {
    const gpa = mr.gpa;
    const r = &context.fr.interface;
    if (r.buffer.len >= capacity) {
        const data = r.buffer[r.seek..r.end];
        @memmove(r.buffer[0..data.len], data);
        r.seek = 0;
        r.end = data.len;
    } else {
        const adjusted_capacity = std.ArrayList(u8).growCapacity(capacity);

        if (r.seek == 0) {
            if (gpa.remap(r.buffer, adjusted_capacity)) |new_memory| {
                r.buffer = new_memory;
                return;
            }
        }

        const data = r.buffer[r.seek..r.end];
        const new = try gpa.alloc(u8, adjusted_capacity);
        @memcpy(new[0..data.len], data);
        gpa.free(r.buffer);
        r.buffer = new;
        r.seek = 0;
        r.end = data.len;
    }
}