Fix blackbody node to support temperatures down to 800K (Fixes #2827)

[zeel2104]

Summary

Fixes #2827

Extends the blackbody node temperature range down to ~800K (the Draper
point) from the previous lower limit of 1667K.

Changes

• GLSL: Lowered clamp from 1667K to 800K. The Kang et al. (2002)
  xc polynomial covers 1000K–4000K with the same formula, so the old
  limit was unnecessarily conservative. Merged the two redundant xc
  branches into one.
• OSL: Replaced manual polynomial with OSL's built-in blackbody()
  • luminance(), which is physically exact with no lower temperature limit.

Testing

Verified the output chromaticity gradient from 800K–6504K is physically
correct and all temperatures below 1667K now return distinct colors
instead of being clamped to the 1667K value.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: zeel2104 / name: Zeel Desai (176e288)