Transparency configuration in Qt EGLFS/KMS application

It is needed to support premultiplied transparent mode on startup for EGLFS/KMS platform-based application. It could be supported if used blend_op property for drmPlane (by default, it used opacity value - coverage).

It would be very useful to be able to initialize the property via environment variable on startup. 

Surely, it can be set manually by DRM interface:

int drmModeAtomicAddProperty(drmModeAtomicReqPtr req, uint32_t object_id,  uint32_t property_id, uint64_t value);

But it might be a problem with single blink on start.