Changes to reduce allocations in the main loop.

Targeted some methods that generated allocated a lot of memory in the main game loop:
- Actor.Render - Changed LINQ calls into equivalent loops. No allocation for delegates.
- Animation.Render - Returned an array rather than a yield expression. The array uses less memory than the complier generated enumerable.
- FrozenActor and FrozenUnderFog - Materialize the footprint into an array: The enumerable is not-trivial to evaluate is and evaluated many times inside the Tick function. The memory needed is minimal. Changed LINQ into equivalent loops to prevent delegate allocation. Should result in overall much faster calls.
- Widget.GetEventBounds - Changed LINQ calls into equivalent loops.
- MobileInfo.CanEnterCell - Changed LINQ calls into equivalent loops. Don't materialize list of blocking actors every time, instead enumerate them and only when they need to be checked.
- FrozenUnderFog.TickRender - Generate the renderables lazily and also remove a no-op Select call.

OpenRA.Game/Graphics/Animation.cs

@@ -49,13 +49,16 @@ namespace OpenRA.Graphics
 
 		public IEnumerable<IRenderable> Render(WPos pos, WVec offset, int zOffset, PaletteReference palette, float scale)
 		{
+			var imageRenderable = new SpriteRenderable(Image, pos, offset, CurrentSequence.ZOffset + zOffset, palette, scale, IsDecoration);
+
 			if (CurrentSequence.ShadowStart >= 0)
 			{
 				var shadow = CurrentSequence.GetShadow(CurrentFrame, facingFunc());
-				yield return new SpriteRenderable(shadow, pos, offset, CurrentSequence.ShadowZOffset + zOffset, palette, scale, true);
+				var shadowRenderable = new SpriteRenderable(shadow, pos, offset, CurrentSequence.ShadowZOffset + zOffset, palette, scale, true);
+				return new IRenderable[] { shadowRenderable, imageRenderable };
 			}
 
-			yield return new SpriteRenderable(Image, pos, offset, CurrentSequence.ZOffset + zOffset, palette, scale, IsDecoration);
+			return new IRenderable[] { imageRenderable };
 		}
 
 		public IEnumerable<IRenderable> Render(WPos pos, PaletteReference palette)