Simplify clear colors (#718)

* Use naming constants instead of numbers for clear colors

* Simplify clear colors logic and add comments

skiko/src/awtMain/kotlin/org/jetbrains/skiko/context/SoftwareContextHandler.kt

 package org.jetbrains.skiko.context
 
-import org.jetbrains.skia.Bitmap
-import org.jetbrains.skia.Canvas
-import org.jetbrains.skia.ColorAlphaType
-import org.jetbrains.skia.ImageInfo
+import org.jetbrains.skia.*
 import org.jetbrains.skiko.Logger
-import org.jetbrains.skia.SurfaceProps
+import org.jetbrains.skiko.OS
 import org.jetbrains.skiko.SkiaLayer
 import org.jetbrains.skiko.hostOs
-import org.jetbrains.skiko.OS
-import java.awt.Transparency
 import java.awt.Color
+import java.awt.Transparency
 import java.awt.color.ColorSpace
-import java.awt.image.BufferedImage
-import java.awt.image.ComponentColorModel
-import java.awt.image.DataBuffer
-import java.awt.image.DataBufferByte
-import java.awt.image.Raster
-import java.awt.image.WritableRaster
+import java.awt.image.*
 
 internal class SoftwareContextHandler(layer: SkiaLayer) : JvmContextHandler(layer) {
-    override val clearColor = if (layer.transparency && hostOs == OS.MacOS) 0 else -1
+    override fun isTransparentBackground(): Boolean {
+        // TODO: why Software rendering has another transparency logic from the begginning
+        return hostOs == OS.MacOS && layer.transparency
+    }
 
     val colorModel = ComponentColorModel(
         ColorSpace.getInstance(ColorSpace.CS_sRGB),

skiko/src/commonMain/kotlin/org/jetbrains/skiko/context/ContextHandler.kt

 package org.jetbrains.skiko.context
 
-import org.jetbrains.skia.BackendRenderTarget
-import org.jetbrains.skia.Canvas
-import org.jetbrains.skia.DirectContext
-import org.jetbrains.skia.Picture
-import org.jetbrains.skia.Surface
+import org.jetbrains.skia.*
 import org.jetbrains.skiko.*
-import org.jetbrains.skiko.RenderException
 
 internal abstract class ContextHandler(
     protected val layer: SkiaLayer,
     private val drawContent: Canvas.() -> Unit
 ) {
-    // TODO can we simplify clearColor logic? is there a reason why SoftwareContextHandler has opposite logic?
-    protected open val clearColor = if (layer.transparency || hostOs == OS.MacOS) 0 else -1
     protected var context: DirectContext? = null
     protected var renderTarget: BackendRenderTarget? = null
     protected var surface: Surface? = null
@@ -48,9 +41,22 @@ internal abstract class ContextHandler(
         }
         initCanvas()
         canvas?.apply {
-            clear(if (layer.fullscreen && hostOs != OS.MacOS) -1 else clearColor)
+            clear(if (isTransparentBackground()) Color.TRANSPARENT else Color.WHITE)
             drawContent()
         }
         flush()
     }
+
+    protected open fun isTransparentBackground(): Boolean {
+        if (hostOs == OS.MacOS) {
+            // MacOS transparency is always supported
+            return true
+        }
+        if (layer.fullscreen) {
+            // for non-MacOS in fullscreen transparency is not supported
+            return false
+        }
+        // for non-MacOS in non-fullscreen transparency provided by [layer]
+        return layer.transparency
+    }
 }

skiko/src/jsMain/kotlin/org/jetbrains/skiko/CanvasRenderer.kt

@@ -95,7 +95,7 @@ abstract class CanvasRenderer constructor(val htmlCanvas: HTMLCanvasElement) {
             redrawScheduled = false
             GL.makeContextCurrent(contextPointer)
             // `clear` and `resetMatrix` make canvas not accumulate previous effects
-            canvas?.clear(-1)
+            canvas?.clear(Color.WHITE)
             canvas?.resetMatrix()
             drawFrame(timestamp)
             surface?.flushAndSubmit()