Refactor: use Span.Fill for block color assignment

Replaced manual for loop with Span.Fill to efficiently assign a color value to the output span in ASTC block decoding. This improves code readability and performance.

Author: ds5678

AssetRipper.TextureDecoder/Astc/AstcDecoder.cs

@@ -66,10 +66,7 @@ private static void DecodeBlock(ReadOnlySpan<byte> input, int blockWidth, int bl
 			if (input[0] == 0xfc && (input[1] & 1) == 1)
 			{
 				ColorRGBA<byte> c = new(input[9], input[11], input[13], input[15]);
-				for (int i = 0; i < blockWidth * blockHeight; i++)
-				{
-					output[i] = c;
-				}
+				output[..(blockWidth * blockHeight)].Fill(c);
 			}
 			else
 			{