Dispose of graphics resources deterministically.

Textures, FrameBuffers and VertexBuffers allocated by the Sdl2 Renderer were only being released via finalizers. This could lead to OpenGL out of memory errors since resources may not be cleaned up in a timely manner. To avoid this, IDisposable has been implemented and transitively applied to classes that use these resources.

As a side-effect some static state is no longer static, particularly in Renderer, in order to facilitate this change and just for nicer design in general.

Also dispose some bitmaps.

OpenRA.Game/Map/MapCache.cs

@@ -21,7 +21,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA
 {
-	public class MapCache : IEnumerable<MapPreview>
+	public sealed class MapCache : IEnumerable<MapPreview>, IDisposable
 	{
 		public static readonly MapPreview UnknownMap = new MapPreview(null, null);
 		readonly Cache<string, MapPreview> previews;
@@ -163,7 +163,13 @@ namespace OpenRA
 				foreach (var p in todo)
 				{
 					// The rendering is thread safe because it only reads from the passed instances and writes to a new bitmap
-					var bitmap = p.CustomPreview ?? Minimap.RenderMapPreview(modData.DefaultRules.TileSets[p.Map.Tileset], p.Map, modData.DefaultRules, true);
+					var createdPreview = false;
+					var bitmap = p.CustomPreview;
+					if (bitmap == null)
+					{
+						createdPreview = true;
+						bitmap = Minimap.RenderMapPreview(modData.DefaultRules.TileSets[p.Map.Tileset], p.Map, modData.DefaultRules, true);
+					}
 					// Note: this is not generally thread-safe, but it works here because:
 					//   (a) This worker is the only thread writing to this sheet
 					//   (b) The main thread is the only thread reading this sheet
@@ -172,7 +178,15 @@ namespace OpenRA
 					//       the next render cycle.
 					//   (d) Any partially written bytes from the next minimap is in an
 					//       unallocated area, and will be committed in the next cycle.
-					p.SetMinimap(sheetBuilder.Add(bitmap));
+					try
+					{
+						p.SetMinimap(sheetBuilder.Add(bitmap));
+					}
+					finally
+					{
+						if (createdPreview)
+							bitmap.Dispose();
+					}
 
 					// Yuck... But this helps the UI Jank when opening the map selector significantly.
 					Thread.Sleep(Environment.ProcessorCount == 1 ? 25 : 5);
@@ -226,5 +240,10 @@ namespace OpenRA
 		{
 			return GetEnumerator();
 		}
+
+		public void Dispose()
+		{
+			sheetBuilder.Dispose();
+		}
 	}
 }