Add descriptions to demos

orx-shapes/src/jvmDemo/kotlin/primitives/DemoRectangleIrregularGrid02.kt

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+package primitives
+
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.extra.color.presets.CORAL
+import org.openrndr.extra.shapes.primitives.column
+import org.openrndr.extra.shapes.primitives.irregularGrid
+import org.openrndr.extra.shapes.primitives.row
+import kotlin.random.Random
+
+/**
+ * Demonstrates how to use `Rectangle.irregularGrid()` to create a grid with varying column widths
+ * and row heights. The widths and heights are specified as a list of 13 `Double` values, each
+ * picked randomly between the values 1.0 and 4.0. This produces two types of columns and two
+ * types of rows only: wide ones and narrow ones.
+ *
+ * The program also demonstrates how to query a `row()` and a `column()` from a `RectangleGrid` instance,
+ * both of which return a `List<Rectangle>`. Both `Rectangle` lists are rendered with translucent
+ * colors, which makes the intersection of the column and the row slightly brighter.
+ *
+ */
+fun main() = application {
+    configure {
+        width = 720
+        height = 720
+    }
+    program {
+        extend {
+            val r = Random(100)
+            val grid = drawer.bounds.irregularGrid(
+                List(13) { listOf(1.0, 4.0).random(r) },
+                List(13) { listOf(1.0, 4.0).random(r) }
+            )
+
+            drawer.fill = null
+            drawer.stroke = ColorRGBa.WHITE
+            drawer.rectangles(grid.flatten())
+
+            drawer.stroke = ColorRGBa.BLACK
+            drawer.fill = ColorRGBa.PINK.opacify(0.5)
+            drawer.rectangles(grid.column(2))
+
+            drawer.fill = ColorRGBa.CORAL.opacify(0.5)
+            drawer.rectangles(grid.row(6))
+        }
+    }
+}