fix(MeshTransmissionMaterial): support stencil buffer in render targets

[fremorie]

Why

MeshTransmissionMaterial samples its refraction from an offscreen buffer that it fills by re-rendering the scene every frame into an FBO created with useFBO. That render target is created without a stencil attachment, so any stencil-based masking (useMask / <Mask>) is silently ignored during the transmission pass - masked-out geometry that's correctly hidden in the main render still leaks through the transmissive surface. This makes MeshTransmissionMaterial incompatible with stencil masks, with no way to opt in.

This PR adds an optional stencilBuffer prop to enable a stencil attachment on the internal buffer(s), so masks are respected inside the refraction.

Minimal reproducible example of the bug	Same example with meshPhysicalMaterial (works as expected)
https://fremorie.github.io/shaders/terrarium-transmission-material	https://fremorie.github.io/shaders/terrarium-physical-material
Code	Code
The mask has no effect: the material's internal render targets have no stencil buffer to honor it.	The stencil mask clips the geometry correctly, because the material renders directly to the default framebuffer (which has a stencil buffer when gl={{ stencil: true }})

What

• Added an optional stencilBuffer?: boolean prop to MeshTransmissionMaterial.
• Forwarded it to the useFBO settings for both the main buffer (fboMain) and the backside buffer (fboBack), so masking works in both backside and non-backside modes.
• Handled useFBO's argument overloading so the setting is applied whether or not a numeric resolution is provided (when resolution is omitted, settings must be passed as the first argument, otherwise the third argument is dropped).
• Default false preserves the current behavior exactly - this is fully backward compatible and opt-in.

Usage:

<MeshTransmissionMaterial stencilBuffer transmission={1} />

Screenshots

Before	After (with stencilBuffer={true})

Checklist

• Documentation updated (example)
• Storybook entry added (example)
• Ready to be merged

[vercel]

@fremorie is attempting to deploy a commit to the Poimandres Team on Vercel.

A member of the Team first needs to authorize it.

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.