wg/gl: add composition blend mode

Set the scene to use Composition mode so that it generates
a pre-composition image, and then normal blend with
destination.

see also: https://github.com/thorvg/thorvg/pull/3621

src/renderer/gl_engine/tvgGlRenderer.cpp

@@ -985,7 +985,7 @@ bool GlRenderer::blend(BlendMethod method)
 
     if (method == mBlendMethod) return true;
 
-    mBlendMethod = method;
+    mBlendMethod = (method == BlendMethod::Composition ? BlendMethod::Normal : method);
 
     return true;
 }

src/renderer/wg_engine/tvgWgRenderer.cpp

@@ -286,7 +286,8 @@ bool WgRenderer::blend(BlendMethod method)
     //TODO: support
     if (method == BlendMethod::Hue || method == BlendMethod::Saturation || method == BlendMethod::Color || method == BlendMethod::Luminosity || method == BlendMethod::HardMix) return false;
 
-    mBlendMethod = method;
+    mBlendMethod = (method == BlendMethod::Composition ? BlendMethod::Normal : method);
+
     return true;
 }