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/Graphics/CursorProvider.cs

@@ -16,11 +16,12 @@ using OpenRA.Primitives;
 
 namespace OpenRA.Graphics
 {
-	public class CursorProvider
+	public sealed class CursorProvider : IDisposable
 	{
 		HardwarePalette palette;
 		Dictionary<string, CursorSequence> cursors;
 		Cache<string, PaletteReference> palettes;
+		readonly SheetBuilder sheetBuilder;
 
 		public static bool CursorViewportZoomed { get { return Game.Settings.Graphics.CursorDouble && Game.Settings.Graphics.PixelDouble; } }
 
@@ -45,10 +46,11 @@ namespace OpenRA.Graphics
 			foreach (var p in nodesDict["Palettes"].Nodes)
 				palette.AddPalette(p.Key, new ImmutablePalette(GlobalFileSystem.Open(p.Value.Value), shadowIndex), false);
 
-			var spriteCache = new SpriteCache(modData.SpriteLoaders, new string[0], new SheetBuilder(SheetType.Indexed));
+			sheetBuilder = new SheetBuilder(SheetType.Indexed);
+			var spriteCache = new SpriteCache(modData.SpriteLoaders, new string[0], sheetBuilder);
 			foreach (var s in nodesDict["Cursors"].Nodes)
 				LoadSequencesForCursor(spriteCache, s.Key, s.Value);
-			spriteCache.SheetBuilder.Current.ReleaseBuffer();
+			sheetBuilder.Current.ReleaseBuffer();
 
 			palette.Initialize();
 		}
@@ -95,5 +97,11 @@ namespace OpenRA.Graphics
 				throw new InvalidOperationException("Cursor does not have a sequence `{0}`".F(cursor));
 			}
 		}
+
+		public void Dispose()
+		{
+			palette.Dispose();
+			sheetBuilder.Dispose();
+		}
 	}
 }