Fix bounds on mask composite

[giantpand2000]

Make the right IntRect from Mask and position

[jrmuizel]

Can you add a test as well?

[giantpand2000]

Added. Previous will not pass this test:

> Executing task: cargo test --package raqote --lib -- tests::tests::mask_composite_position --exact --nocapture <

    Finished test [unoptimized + debuginfo] target(s) in 0.07s
     Running unittests src/lib.rs (target/debug/deps/raqote-697e8df342672d23)

running 1 test
thread 'tests::tests::mask_composite_position' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `4294901760`', src/tests.rs:998:9