16666484525a4f0c90be4e5cbb077c9b679c7b94  [WebGPU] https://playground.babylonjs.com/#18WTCK doesn't render trees and stones

Classified as not security-relevant.
Triage note: Adds PixelFormat::RGBA16 across PixelBuffer/PixelFormat/PixelBufferConversion/ImageBuffer/IOSurface/WebGL gated on ENABLE(PIXEL_FORMAT_RGBA16) (with an #error for an unimplemented path); net-new format feature, not a fix.
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), bounds_guard (+3.25), null_guard (+2.59), area:Source/WebCore/html (+2.0), large_churn (-2.0)