Add blend modes to renderer.

Voile/Source/Rendering/RenderSystem.cs

@@ -2,6 +2,38 @@ using System.Numerics;
 
 namespace Voile.Rendering
 {
+    public enum BlendMode
+    {
+        //
+        // Summary:
+        //     Blend textures considering alpha (default)
+        BlendAlpha,
+        //
+        // Summary:
+        //     Blend textures adding colors
+        BlendAdditive,
+        //
+        // Summary:
+        //     Blend textures multiplying colors
+        BlendMultiplied,
+        //
+        // Summary:
+        //     Blend textures adding colors (alternative)
+        BlendAdd,
+        //
+        // Summary:
+        //     Blend textures subtracting colors (alternative)
+        BlendSubtract,
+        //
+        // Summary:
+        //     Blend premultiplied textures considering alpha
+        BlendAlphaPremul,
+        //
+        // Summary:
+        //     Blend textures using custom src/dst factors (use rlSetBlendMode())
+        BlendCustom
+    }
+
     /// <summary>
     /// An abstract class representing the graphics renderer.
     /// </summary>
@@ -82,6 +114,9 @@ namespace Voile.Rendering
         /// </summary>
         public abstract void EndFrame();
 
+        public abstract void BeginBlended(BlendMode blendMode);
+        public abstract void EndBlended();
+
         public abstract void BeginCamera2d(Vector2 offset, Vector2 target, float rotation, float zoom);
         public abstract void EndCamera2d();