LibAccelGfx: Add method to make context active

In the upcoming changes, the AccelGfx context will be used for WebGL, so
we can no longer assume that the WebContent process has a single global
context.

Userland/Libraries/LibAccelGfx/Context.h

@@ -25,6 +25,12 @@ public:
     Context()
     {
     }
+
+    virtual ~Context()
+    {
+    }
+
+    virtual void activate() = 0;
 };
 
 }