wl: Use eglGetPlatformDisplay whenever possible

Plainly using `eglGetDisplay(EGL_DEFAULT_DISPLAY)` can be problematic in certain setups. Similar improvements have been done in other projects before, for example:

* [Patch for WebKitGTK+](https://bugs.webkit.org/show_bug.cgi?id=163333).
* [Patch for Qt's eglfs_kms](https://bugreports.qt.io/browse/QTBUG-60330).
* [Pull request for RetroArch](https://github.com/libretro/RetroArch/pull/4826).