Fix draw clipping for projecting containers

Author: WilliamQiufeng

Wobble/Graphics/Container.cs

@@ -20,6 +20,10 @@ public class Container : Drawable
         /// </summary>
         public RenderTargetOptions RenderTargetOptions { get; } = new RenderTargetOptions();
 
+        protected override RectangleF ChildDrawRectangleMask =>
+            RenderTargetOptions.RenderTarget.Value?.Bounds 
+            ?? base.ChildDrawRectangleMask;
+
         /// <summary>
         ///     A projection sprite that has the same dimension, position, rotation and parent as the container.
         ///     It shows <see cref="RenderTarget"/>, which the container can render its entire content to

Wobble/Graphics/Drawable.cs

@@ -76,6 +76,19 @@ public Drawable Parent
         /// </summary>
         public RectangleF ScreenRectangle { get; private set; } = new RectangleF();
 
+        /// <summary>
+        ///     If outside this region, this will not be drawed.
+        /// </summary>
+        private RectangleF DrawRectangleMask { get; set; } =
+            new RectangleF(0, 0, WindowManager.Width, WindowManager.Height);
+
+        /// <summary>
+        ///     Clipping region for children. Useful to RenderTargets
+        /// </summary>
+        protected virtual RectangleF ChildDrawRectangleMask =>
+            Parent?.ChildDrawRectangleMask
+            ?? new RectangleF(0, 0, WindowManager.Width, WindowManager.Height);
+
         /// <summary>
         ///     The bounding box of the drawable relative to the entire screen.
         /// </summary>
@@ -584,6 +597,8 @@ protected void RecalculateRectangles()
             // Update AbsoluteRotation
             AbsoluteRotation = (Parent?.AbsoluteRotation ?? 0) + Rotation;
             AbsoluteScale = (Parent?.AbsoluteScale ?? Vector2.One) * Scale;
+            DrawRectangleMask = Parent?.ChildDrawRectangleMask
+                                ?? new RectangleF(0, 0, WindowManager.Width, WindowManager.Height);
 
             // Make it relative to the parent.
             var width = RelativeWidth;