Change the handling of out-of-bounds canvas

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

@@ -23,8 +23,6 @@ internal interface TextCanvas {
 	operator fun get(row: Int, column: Int): TextPixel
 }
 
-private val blankPixel = TextPixel(' ')
-
 internal class TextSurface(
 	override val width: Int,
 	override val height: Int,
@@ -33,16 +31,23 @@ internal class TextSurface(
 	override var translationX = 0
 	override var translationY = 0
 
-	private val rows = Array(height) { Array(width) { TextPixel(' ') } }
+	private val rows = Array(height) { Array(width) { newBlankPixel } }
 
-	override operator fun get(row: Int, column: Int) = rows[translationY + row][translationX + column]
+	override operator fun get(row: Int, column: Int): TextPixel {
+		val x = translationX + column
+		val y = translationY + row
+		if (x >= width || y >= height || x < 0 || y < 0) {
+			return reusableDirtyPixel
+		}
+		return rows[y][x]
+	}
 
 	fun appendRowTo(appendable: Appendable, row: Int) {
 		// Reused heap allocation for building ANSI attributes inside the loop.
 		val attributes = mutableListOf<Int>()
 
 		val rowPixels = rows[row]
-		var lastPixel = blankPixel
+		var lastPixel = reusableBlankPixel
 		for (columnIndex in 0 until width) {
 			val pixel = rowPixels[columnIndex]
 
@@ -153,6 +158,23 @@ internal class TextSurface(
 	}
 }
 
+/**
+ * Returns always a new blank [TextPixel].
+ */
+private val newBlankPixel: TextPixel get() = TextPixel(' ')
+
+/**
+ * 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
+
+/**
+ * It is used in places where the [TextPixel] state is not important
+ * and it can change.
+ */
+private val reusableDirtyPixel: TextPixel = newBlankPixel
+
 internal class TextPixel(var codePoint: Int) {
 	var background: Color = Color.Unspecified
 	var foreground: Color = Color.Unspecified