qt: try to take care of eglfs_kms platform plugin

VLC supports KMS, Qt supports KMS. And DRM/KMS itself supports overlaying when applicable. With this merge request, I tried to add basic support without custom composition.

I have tried both;

• eglfs_kms
• eglfs_kms_egldevice (with Nvidia)

Eglfs_kms runs, but overlaying is not supported by Qt at the moment. See https://bugreports.qt.io/browse/QTBUG-62247. Eglfs_kms_egldevice does not work for me, but it supports overlay according to the sources and the documentation.

However, there are more integrations that target specific hardware. In these cases, overlaying should work as long as it is supported.

Most configuration is done via environment variables. For example, at least these should be set:

• QT_QPA_EGLFS_KMS_ATOMIC=1
• QT_QPA_EGLFS_LAYER_INDEX=1