fix: SG-43605: OTIO export failing when OCIO is active, and support reversed-order stack blending

[markreidvfx]

Summarize your change.

Fix OTIO export failing when OCIO is active, and support reversed-order stack blending.

When OCIO is enabled, the RVLinearize node is not present and the CDL parameters are instead located on an RVColor node. The export hook calls associatedNode("RVLinearize"), which returns an empty string, and the subsequent call to commands.getIntProperty("{}.CDL.active".format(linearize)) then raises an exception.

This fix loops over both RVLinearize and RVColor nodes, checking propertyExists before accessing CDL properties.

Also reverses the input ordering when exporting an RVStack with supportReversedOrderBlending enabled, so the OTIO layer order matches RV.

Describe the reason for the change.

OTIO export throws this exception when OCIO is active:

File ".../sourcePostExportHook.py", line 21, in hook_function
    if commands.getIntProperty("{}.CDL.active".format(linearize))[0] != 0:
Exception: "invalid property name .CDL.active", program exception

Describe what you have tested and on which operating system.

Tested on Rocky Linux 9.

1. Load media with OCIO active
2. Activate a CDL on RVColor:

from rv import commands, extra_commands
source = commands.nodesOfType("RVSource")[0]
color = extra_commands.associatedNode("RVColor", source)
commands.setIntProperty(f"{color}.CDL.active", [1], True)
commands.setFloatProperty(f"{color}.CDL.slope", [1.1, 0.9, 1.0], True)

3. File > Export > OTIO — fails with "invalid property name .CDL.active"