BTerm::left_click is set to `true` at button press and release

As the title says, `BTerm::left_click` is set twice per mouse click (press down and release), which leads to mouse-based events being fired twice in a row. I discovered this because toggles based on mouse clicks are being activated and deactivated again immediately.

I don't know if this is intended behaviour and it can be worked around easily, but it's nevertheless counter-intuitive.

Tested and verified with rltk 0.8.1 and bracket-lib tutorial chapter 74 by adding a check for `ctx::left_click` in the function

```rust
fn player_input(gs: &mut State, ctx: &mut Rltk) -> RunState {...}
```