Fixes colMul premultiplied color in FSprites (#1448)

Fixes colMul premultiplied color

korge/src/commonMain/kotlin/com/soywiz/korge/view/fast/FSprites.kt

@@ -229,7 +229,7 @@ open class FSprites(val maxSize: Int) {
                 val baseSize = TEMP(VarType.Float2)
                 val texSize = TEMP(VarType.Float2)
                 SET(baseSize, a_uv1 - a_uv0)
-                SET(v_Col, a_colMul)
+                SET(v_Col, vec4(a_colMul["rgb"] * a_colMul["a"], a_colMul["a"])) // Pre-multiply color here
                 SET(v_TexId, a_texId)
 
                 //SET(texSize, u_i_texSizeN[0])

korge/src/jvmTest/kotlin/com/soywiz/korge/view/fast/FSpritesJvmTest.kt

@@ -2,7 +2,9 @@ package com.soywiz.korge.view.fast
 
 import com.soywiz.korge.testing.*
 import com.soywiz.korge.tests.*
+import com.soywiz.korge.view.*
 import com.soywiz.korim.bitmap.*
+import com.soywiz.korim.color.*
 import com.soywiz.korma.geom.*
 import kotlin.test.*
 
@@ -25,4 +27,31 @@ class FSpritesJvmTest : ViewsForTesting(log = true) {
         }
         assertScreenshot(this, includeBackground = true)
     }
+
+    @Test
+    fun testAlpha() = korgeScreenshotTest(SizeInt(20, 20), bgcolor = Colors.GREEN) {
+        val bmp = Bitmap32(10, 10, Colors.RED).premultiplied()
+        image(bmp).xy(0, 0)
+        image(bmp) {
+            xy(0, 10)
+            alpha = .5
+        }
+
+        val fSprites = FSprites(2)
+        addChild(fSprites.createView(bmp))
+        with(fSprites) {
+            val left = fSprites.alloc()
+            left.xy(10f, 0f)
+            left.colorMul = Colors.WHITE
+            left.setTexIndex(0)
+            left.setTex(bmp.slice())
+
+            val right = fSprites.alloc()
+            right.xy(10f, 10f)
+            right.colorMul = Colors.WHITE.withAf(.5f)
+            right.setTexIndex(0)
+            right.setTex(bmp.slice())
+        }
+        assertScreenshot(this, includeBackground = true)
+    }
 }