linux: fix parsing of xrandr output for rare screen configurations

This is the first of two fixes related to [MC-70651](https://bugs.mojang.com/browse/MC-70651).
I created two PRs for this because one patch is just a fix for lwjgl, the other one a workaround for dubious driver behavior.

This is the simple fix. LWJGL fails to parse xrandr output in some cases:

The code assumed frequencies can be directly followed by a star or a plus sign (\*,+) to indicate the preferred and the current frequency. It turns out xrandr generates an additional space char if the mode is preferred (+), but not current (*). Expected vs. actual:
```
1920*1080 60.0+ 50.0*
1920*1080 60.0 + 50.0*
```
Also, the old code failed to detect the current screen configuration in some cases were it was not the first one in the list.

This PR fixes both. This should not have any negative impact.