If press/release events for a mouse button come too fast, then using
`IsMouseButtonReleased()` does not work. This has been noticed when
using a touchpad on Linux when tapping with two fingers two emulate
right mouse button click.

The situation looks like this:

```
BeginDrawing           <-- current==released, previous==released
Pressed                <-- current=pressed
Released               <-- current=released
IsMouseButtonReleased  <-- returns false because current==previous
EndDrawing             <-- previous=released
```

The fix is to update the previous mouse button state in addition to
current mouse button state when `MouseButtonCallback()` is called by
glfw. Now the situation is as follows:

```
BeginDrawing           <-- current==released, previous==released
Pressed                <-- current=pressed, previous=released
Released               <-- current=released, previous=pressed
IsMouseButtonReleased  <-- returns true because current!=previous
EndDrawing             <-- previous=released
```