eglfs/kms improvements for overlay planes, modesetting, and others

Focus on eglfs and DRM (just basic overlay planes at first in eglfs_kms (GBM), then maybe add dmabuf import in the second phase; maybe something for EGLStream/EGLLayer for NVIDIA later). Expose a semi-private, eglfs-specific layer (plane) management interface via platformheaders and QEglFSFunctions, and iterate in order to get an idea for what is sensible for public APIs in future Qt versions.

Note: QTBUG-58690 is expected to provide an independent solution for Renesas systems, introducing a eglfs_kms_vsp2 backend that offers dmaBuf import and composing those via V4L/VSP.