[40] backends/native: Ensure pointer is onscreen in input thread
It's slightly racy to have the main thread update the views and warp the pointer, since the input thread may not be aware yet of the new viewport layout.
Make the input thread clamp the pointer so it remains onscreen instead, when the new viewports are obtained.
Closes: #64 (closed) Part-of: !2237 (merged)
(cherry picked from commit 70e44c28)