RayPortalMaterialData: Use albedo texture as portal mask

[xoxor4d]

This small change allows the usage of the albedo texture to define the rayportal shape when creating a rayportal material via the remixApi.

Background:
The shape of the portal gets estimated if MaskTexture2 is unset.
This results in a perfect circle but that might not always be the desired shape.
In case of portal 2, I actually need square portals for room/area connecting portals that are placed within sliding doors or similar.
Using a blank white texture results in just that. Not setting an albedo means that the portal shape will be estimated just like before.

Note:
I think it might be beneficial to expose that setting to usd/toolkit. From what I can tell it's already hooked up in code .. just unused:

#define LIST_PORTAL_MATERIAL_TEXTURES(X) \
  /*Parameter Name, USD Token String,       Type,       UNUSED...   Default Value */ \
  X(MaskTexture,    emissive_mask_texture,  TextureRef, void, void, {}) \
  X(MaskTexture2,   unused_in_usd_so_dont,  TextureRef, void, void, {})

dxvk-remix/src/dxvk/rtx_render/rtx_scene_manager.cpp

Line 1096 in 912a164

trackTexture(ctx, rayPortalMaterialData.getMaskTexture2(), maskTextureIndex2, hasTexcoords, false);