videoprovider: cleanup synchronization with the scene graph renderer

Applies some suggestions based on reviews by vstoiakin regarding concurrent resource access and overall structural simplification.
- Create and handoff ownership of QVideoFrames each frame
- use QReadWriteLocks when multiple reads are possible instead of QMutex
- remove the superfluous layer of unique_ptrs on QVideoFrames which are explicitly shared using QExplicitlySharedDataPointer for their underlying data.

This patch is required to prevent crashes when using Qt 6.4 and up.

Gitlab: #938
Change-Id: Ia9a9f0310344bcee47dbd88ec622bc3daa2ce6d7