Change the handling of out-of-bounds canvas

mosaic-runtime/src/commonMain/kotlin/com/jakewharton/mosaic/surface.kt

@@ -18,6 +18,11 @@ import de.cketti.codepoints.appendCodePoint
 
 private val blankPixel = TextPixel(' ')
 
+/**
+ * Returns always a new blank [TextPixel].
+ */
+private inline fun newBlankPixel(): TextPixel = TextPixel(' ')
+
 public interface TextCanvas {
 	public val height: Int
 	public val width: Int
@@ -35,19 +40,32 @@ internal class TextSurface(
 
 	private val cells = Array(width * height) { TextPixel(' ') }
 
+	/**
+	 * It is used in places where the [TextPixel] state is not important
+	 * and it can change.
+	 */
+	private val reusableDirtyPixel: TextPixel = newBlankPixel()
+
+	/**
+	 * It is used in places where it is important that the [TextPixel]
+	 * has its original state and **will not change**.
+	 */
+	private val reusableBlankPixel: TextPixel = newBlankPixel()
+
 	operator fun get(row: Int, column: Int): TextPixel {
 		val x = translationX + column
 		val y = row + translationY
-		check(x in 0 until width)
-		check(y in 0 until height)
+		if (x >= width || y >= height || x < 0 || y < 0) {
+			return reusableDirtyPixel
+		}
 		return cells[y * width + x]
 	}
 
 	override fun appendRowTo(appendable: Appendable, row: Int, ansiLevel: AnsiLevel) {
 		// Reused heap allocation for building ANSI attributes inside the loop.
 		val attributes = mutableIntListOf()
 
-		var lastPixel = blankPixel
+		var lastPixel = reusableBlankPixel
 
 		val rowStart = row * width
 		val rowStop = rowStart + width