BlendMode
PDF blend modes (ISO 32000-1 §11.3.5).
16 standard modes: 12 separable (each colour channel computed independently) and 4 non-separable (operate on HSL). The renderer maps these to the backend's native blend modes — Compose Multiplatform exposes most of them via androidx.compose.ui.graphics.BlendMode.
Unknown / non-blending modes fall back to Normal.