Added basic color constants into Color object (#207)

samples/SkiaNativeSample/src/nativeMain/kotlin/org.jetbrains.skiko.sample.native/App.kt

@@ -50,13 +50,9 @@ fun displayScene(renderer: Renderer, nanoTime: Long) {
     val canvas = renderer.canvas!!
 
     val paint = Paint()
-    // TODO: make color constants.
-    //paint.setColor(Color.GREEN)
-    paint.setColor(0xFF00FF00.toInt())
+    paint.setColor(Color.GREEN)
 
-    // TODO: make color constants.
-    // canvas.clear(Color.RED)
-    canvas.clear(0xFFFF0000.toInt())
+    canvas.clear(Color.RED)
 
     canvas.save();
     canvas.translate(128.0f, 128.0f)

skiko/src/commonMain/kotlin/org/jetbrains/skia/Color.kt

@@ -59,4 +59,14 @@ object Color {
         require(0 <= b && b <= 255) { "Blue is out of 0..255 range: $b" }
         return b and 0xFF or (color and -0x100)
     }
+
+    const val TRANSPARENT: Int   = 0x00_00_00_00
+    const val BLACK: Int         = 0xFF_00_00_00.toInt()
+    const val WHITE: Int         = 0xFF_FF_FF_FF.toInt()
+    const val RED: Int           = 0xFF_FF_00_00.toInt()
+    const val GREEN: Int         = 0xFF_00_FF_00.toInt()
+    const val BLUE: Int          = 0xFF_00_00_FF.toInt()
+    const val YELLOW: Int        = 0xFF_FF_FF_00.toInt()
+    const val CYAN: Int          = 0xFF_00_FF_FF.toInt()
+    const val MAGENTA: Int       = 0xFF_FF_00_FF.toInt()
 }
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