Fix thread-safe access for render entities

[heinezen]

The current render entity code was not thread-safe because of an oversight in the implementation. When syncing data between the render entity and the objects in the renderer, sometimes references were passed instead of copying the data over, e.g.

openage/libopenage/renderer/stages/world/render_entity.cpp

Lines 63 to 67 in 66098ce

	const curve::Continuous<coord::scene3> &WorldRenderEntity::get_position() {
	std::shared_lock lock{this->mutex};
	return this->position;
	}

After returning the reference, the lock is released even though a read has not taken place yet. This results in the simulation thread writing data into the render entity while it is still being read, with the consequence that over- or underreads might happen. I fixed this behavior now by ensuring that locks are present for the full read duration.

Changes in this PR:

• Document which methods of the render entities are thread-safe in the docstrings
• Add a new method get_read_lock() which returns a std::shared_lock locking the render entity's mutex. This allows manual control of data reads from the render entity.
• Add a renderer::RenderEntity base class for the shared functionality of the different render entity types in HUD, terrain, and world render stages.
• Make all data transfers thread-safe

[TheJJ]

great! less code duplication now, and better locking :)