Trim memory usage of Png.

For indexed PNGs, we only need to allocate a palette large enough to accommodate the number of indexed colours in the image. For images that don't use all 256 colours, this slightly reduces the memory usage for these images.

OpenRA.Game/FileFormats/Png.cs

@@ -94,8 +94,8 @@ public Png(Stream s)
 
 						case "PLTE":
 						{
-							Palette = new Color[256];
-							for (var i = 0; i < length / 3; i++)
+							Palette = new Color[length / 3];
+							for (var i = 0; i < Palette.Length; i++)
 							{
 								var r = ms.ReadUInt8(); var g = ms.ReadUInt8(); var b = ms.ReadUInt8();
 								Palette[i] = Color.FromArgb(r, g, b);