Resolve "enh(viz): layer should follow the render mode policy"

Description

• Created a separate render:::render_now() function slot that triggers a render only in manual mode.
• Deprecated render_request slot. The function is kept but does nothing in manual mode, it only really requests a render in auto mode.
• adaptor::render_request() and layer::render_request() become simple shortcuts to render:::render_request().

Closes #1202 (closed)

How to test it?

Test applications that use the manual mode, such as tuto17, ex_video_tracking, etc... But also applications in auto mode like sight_viewer.