Use new video render context API in Display

[aleksandr-voitenko]

⚠️ This PR depends on streamlabs/obs-studio#720 (Fix video mix canvas identity)

Description

This change updates display rendering in obs-studio-node to use the new mix-based video render context introduced in obs-studio.

Instead of setting only the current canvas before preview rendering, DisplayCallback() now:

• resolves the active video mix from (canvas, rendering mode)
• sets that mix as the current render context
• sets the rendering mode before any source rendering begins

This replaces the old canvas-only setup:

• removes the call to obs_set_video_rendering_canvas(dp->m_canvas)
• replaces it with obs_set_video_render_context(obs_video_mix_get(...))

The callback also no longer applies obs_set_video_rendering_mode() only for transition sources. Since the render mode is now set up front for the entire display callback, the old transition-only branch is no longer needed.

Motivation and Context

This aligns obs-studio-node preview/display rendering with the updated obs-studio render-context model.

How Has This Been Tested?

Manually. Windows only.

Types of changes

• Bug fix (non-breaking change which fixes an issue)