Fix code that could lead to a possible deadlock. (#1380)

* Fix code that could lead to a possible deadlock.

Drop implementations are not called until the end of a statement. The statement changed in this commit therefore took 4 read locks on a RwLock which can lead to problems if a write lock is requested between any of these read locks. The code looks like it would only hold one lock at a time but it does not drop any of the locks until after the arithmatic operations complete, which leads to this situation. See https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=996079046184329f3a9df1cd19c87da8 to see this in action. The fix is to just take one lock and share it between the three calls to num_presses, letting it drop at the end of the scope.

* Fix code that may cause a deadlock in `MenuRoot::stationary_interaction`

The issue here is related to that in 9673b8f2a0 in that the lock is not dropped when it is expected.  Since the `RwLockReadGuard` produced by `ctx.input()` has a reference taken from it (one into `pointer`), the lock cannot be dropped until that reference is no longre valid, which is the end of the scope (in this case this function).  The following `ctx.input()` then attempts to aquire a second lock on the `RwLock`, creating the same situation that was found in the referenced commit.

This has been resolved by holding one lock on the input for the whole function.

* Reference this PR from comments in the code for future maintainers

* Add the change to the changelog

* Use full link to PR

* Use full link to PR

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

CHANGELOG.md

@@ -17,7 +17,7 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 
 ### Fixed 🐛
 * Fixed ComboBoxes always being rendered left-aligned ([#1304](https://github.com/emilk/egui/pull/1304)).
-
+* Fixed ui code that could lead to a deadlock ([#1380](https://github.com/emilk/egui/pull/1380))
 
 ## 0.17.0 - 2022-02-22 - Improved font selection and image handling