api: Add Composition blend mode

The composition mode enables intermediate blending. When a scene
 uses Composition mode, it generates an intermediate buffer to
render the scene image first. This image can then be blended
with the canvas afterward.

Please avoid to use this, unless you really need to composite
the precomposite scene. This feature is relatively expensive
at performance.

C++ API:
+BlendMethod::Composition

C API:
+Tvg_Blend_Method::TVG_BLEND_METHOD_COMPOSITION

inc/thorvg.h

@@ -214,7 +214,8 @@ enum class BlendMethod : uint8_t
     Color,             ///< Reserved. Not supported.
     Luminosity,        ///< Reserved. Not supported.
     Add,               ///< Simply adds pixel values of one layer with the other. (S + D)
-    HardMix            ///< Reserved. Not supported.
+    HardMix,           ///< Reserved. Not supported.
+    Composition = 255  ///< Used for intermediate composition. @since 1.0
 };
 
 

src/bindings/capi/thorvg_capi.h

@@ -164,7 +164,8 @@ typedef enum {
     TVG_BLEND_METHOD_COLOR,             ///< Reserved. Not supported.
     TVG_BLEND_METHOD_LUMINOSITY,        ///< Reserved. Not supported.
     TVG_BLEND_METHOD_ADD,               ///< Simply adds pixel values of one layer with the other. (S + D)
-    TVG_BLEND_METHOD_HARDMIX            ///< Reserved. Not supported.
+    TVG_BLEND_METHOD_HARDMIX,           ///< Reserved. Not supported.
+    TVG_BLEND_METHOD_COMPOSITION = 255  ///< Used for intermediate composition. @since 1.0
 } Tvg_Blend_Method;
 
 

src/renderer/sw_engine/tvgSwRenderer.cpp

@@ -528,6 +528,7 @@ bool SwRenderer::blend(BlendMethod method)
 
     switch (method) {
         case BlendMethod::Normal:
+        case BlendMethod::Composition:
             surface->blender = nullptr;
             break;
         case BlendMethod::Multiply:

src/renderer/tvgPaint.cpp

@@ -429,11 +429,14 @@ uint8_t Paint::opacity() const noexcept
 
 Result Paint::blend(BlendMethod method) noexcept
 {
-    if (method > BlendMethod::HardMix) return Result::InvalidArguments;
     //TODO: Remove later
     if (method == BlendMethod::Hue || method == BlendMethod::Saturation || method == BlendMethod::Color || method == BlendMethod::Luminosity || method == BlendMethod::HardMix) return Result::NonSupport;
-    pImpl->blend(method);
-    return Result::Success;
+
+    if (method == BlendMethod::Composition || method <= BlendMethod::HardMix) {
+        pImpl->blend(method);
+        return Result::Success;
+    }
+    return Result::InvalidArguments;
 }