EGLDevice backends needs better configurability

Based on customer experience with various NVIDIA embedded boards, the EGLDevice/EGLOutput/EGLStream backend of eglfs could benefit from having more environment variables that can be used to fix up things on some boards that only care about a single screen, interact with other EGLStream producers, and possibly have a questionable configuration wrt overlay planes.

In addition to the existing QT_QPA_EGLFS_LAYER_INDEX, we should ideally be able to also override:

• the stream FIFO length
• the connector index (as a shortcut to doing this via the JSON config)
• request the usage of a plane (drmModeSetPlane...)