Feature request: Add option to use system packages instead of auto-installing dependencies for tile merge step

[ZainAallii]

Describe the problem

The tile merge step in tile_merge.py currently auto-installs Pillow and OpenEXR via pip
into a temp directory (deadline_pip_packages) at runtime on the farm worker. This was
necessary because V-Ray is installed via host config script rather than Conda, so Conda
packages cannot be relied upon for merge step dependencies.

This approach works but has downsides:

• Unexpected pip install at render time may surprise studio pipeline teams
• Requires internet access on farm workers
• Bypasses studio-managed package environments

Proposed Solution

Add an optional checkbox in the V-Ray tab: "Auto-install merge dependencies (Pillow/OpenEXR)".

• If checked (default): current behavior — pip installs to temp dir if packages not found
• If unchecked: skip auto-install, use whatever is on the system. Fail with a clear error
  if Pillow/OpenEXR are not available.

This gives studios control over their package environment while keeping the current
behavior as the default for ease of use.

Example Use Cases

1. Studio has Pillow/OpenEXR pre-installed on all farm workers via their own pipeline
   setup — they want to disable auto-install to avoid unexpected network calls
2. Farm workers have no internet access — auto-install would fail silently,
   explicit error is better
3. Studio uses Conda and wants to manage all dependencies through their environment

Related

• PR feat: add V-Ray export workflow with tile rendering #217 (V-Ray Standalone workflow)
• Discussion with leongdl feat: add V-Ray export workflow with tile rendering #217 (comment)