Use more locking to improve thread-safety in Sheet.

OpenRA.Game/Graphics/Sheet.cs

@@ -72,6 +72,8 @@ namespace OpenRA.Graphics
 		}
 
 		void GenerateTexture()
+		{
+			lock (textureLock)
 			{
 				if (texture == null)
 				{
@@ -81,8 +83,7 @@ namespace OpenRA.Graphics
 
 				if (data != null)
 				{
-				lock (textureLock)
-				{
+
 					if (dirty)
 					{
 						texture.SetData(data, Size.Width, Size.Height);
@@ -139,6 +140,8 @@ namespace OpenRA.Graphics
 		}
 
 		public void CreateBuffer()
+		{
+			lock (textureLock)
 			{
 				if (data != null)
 					return;
@@ -148,6 +151,7 @@ namespace OpenRA.Graphics
 					data = texture.GetData();
 				releaseBufferOnCommit = false;
 			}
+		}
 
 		public void CommitData()
 		{