Trim memory usage of SpriteCache.

As the SpriteCache is used as a one-shot operation in practise, holding on to the capacity of backing collections is not required. Memory usage can be reduced by allowing the capacity to be reset after the SpriteCache has resolved items.

- Once LoadReservations is called, reset the reservation dictionaries so their backing collections can be reclaimed.
- When ResolveSprites is called, shrink the resolved dictionary as resolutions take place.

OpenRA.Game/Graphics/SpriteCache.cs

@@ -112,6 +112,11 @@ namespace OpenRA.Graphics
 				}
 			}
 
+			spriteReservations.Clear();
+			spriteReservations.TrimExcess();
+			reservationsByFilename.Clear();
+			reservationsByFilename.TrimExcess();
+
 			// When the sheet builder is adding sprites, it reserves height for the tallest sprite seen along the row.
 			// We can achieve better sheet packing by keeping sprites with similar heights together.
 			var orderedPendingResolve = pendingResolve.OrderBy(x => x.Frame.Size.Height);
@@ -137,17 +142,17 @@ namespace OpenRA.Graphics
 				modData.LoadScreen?.Display();
 			}
 
-			spriteReservations.Clear();
-			reservationsByFilename.Clear();
-
 			foreach (var sb in SheetBuilders.Values)
 				sb.Current.ReleaseBuffer();
 		}
 
 		public Sprite[] ResolveSprites(int token)
 		{
-			var resolved = resolvedSprites[token];
-			resolvedSprites.Remove(token);
+			if (!resolvedSprites.Remove(token, out var resolved))
+				throw new InvalidOperationException($"{nameof(token)} {token} has either already been resolved, or was never reserved via {nameof(ReserveSprites)}");
+
+			resolvedSprites.TrimExcess();
+
 			if (missingFiles.TryGetValue(token, out var r))
 				throw new FileNotFoundException($"{r.Location}: {r.Filename} not found", r.Filename);