Exposing gdal_sys types from gdal

[lnicola]

Someone was asking on Discord about retrieving a Dataset's GCPs. This is pretty easy to implement:

    #[cfg(major_ge_3)]
    /// Fetch GCPs
    pub fn gcps(&self) -> &[gdal_sys::GDAL_GCP] {
        let len = unsafe { gdal_sys::GDALGetGCPCount(self.c_dataset) };
        let data = unsafe { gdal_sys::GDALGetGCPs(self.c_dataset) };
        unsafe { std::slice::from_raw_parts(data, len as usize) }
    }

But I couldn't find another place where we're exposing gdal-sys from the gdal API. That might be wrong, though.

The problem is that if we make a new type, we'll either have to allocate a Vec or transmute the pointer. I'd rather avoid the allocation, what do y'all think?

[lnicola]

Actually there are some strings in there, which I've missed. We can make a copy instead.