Fix compilation on recent Rust versions: complete mutable borrows

Compilation of this repo on rustc 1.80.0 used to fail with:

error[E0499]: cannot borrow `target` as mutable more than once at a time
   --> src/map.rs:300:25
    |
295 |         let mut piet_context = target.render_context();
    |                                ----------------------- first mutable borrow occurs here
...
300 |         let image_buf = target.to_image_buf(ImageFormat::RgbaPremul).unwrap();
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ second mutable borrow occurs here
...
311 |     }
    |     - first borrow might be used here, when `piet_context` is dropped and runs the `Drop` code for type `CoreGraphicsContext`

error[E0499]: cannot borrow `target` as mutable more than once at a time
    --> src/map.rs:1367:41
     |
1312 |                         let mut piet_context = target.render_context();
     |                                                ----------------------- first mutable borrow occurs here
...
1367 |                         let image_buf = target.to_image_buf(ImageFormat::RgbaPremul).unwrap();
     |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ second mutable borrow occurs here
...
1370 |                     });
     |                     - first borrow might be used here, when `piet_context` is dropped and runs the `Drop` code for type `CoreGraphicsContext`

error[E0499]: cannot borrow `target` as mutable more than once at a time
    --> src/map.rs:1400:41
     |
1379 |                         let mut piet_context = target.render_context();
     |                                                ----------------------- first mutable borrow occurs here
...
1400 |                         let image_buf = target.to_image_buf(ImageFormat::RgbaPremul).unwrap();
     |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ second mutable borrow occurs here
...
1403 |                     });
     |                     - first borrow might be used here, when `piet_context` is dropped and runs the `Drop` code for type `CoreGraphicsContext`

This PR includes changes to ensure the mutable borrows are complete before subsequent calls in order to run without issues on modern Rust versions.

spstreets / gtfs_manager

Fix compilation on recent Rust versions: complete mutable borrows #60