Rgba Texture
Texture backed by a tightly-packed, top-row-first RGBA8 byte array — the output of the KTX2 / Basis-Universal transcoder (KHR_texture_basisu), which has no platform image object to wrap. Mirrors ImageTexture's vertical flip and mipmap policy so basisu textures line up with glTF UVs exactly like the JPEG/PNG path, and premultiplies alpha to match the engine's premultiplied blend (GL_ONE, GL_ONE_MINUS_SRC_ALPHA).
Unsized GL_RGBA internal format keeps it valid on WebGL1 / GLES2.