Set minimum size of Java2D window

Just setting size will be ignored in the content is smaller than this.
Setting the minimum size forces the window to have at least the given
size.

Fixes #179

java2d/src/main/scala/doodle/java2d/effect/Java2dWindow.scala

@@ -20,6 +20,7 @@ import doodle.core.Point
 import doodle.java2d.Picture
 import doodle.java2d.effect.Size.FixedSize
 
+import java.awt.Dimension
 import java.awt.event.ActionEvent
 import java.awt.event.ActionListener
 import java.awt.event.WindowAdapter
@@ -99,7 +100,7 @@ final class Java2dWindow(
   getContentPane().add(panel)
   frame.size match {
     case FixedSize(width, height) =>
-      setSize(width.toInt, height.toInt)
+      setMinimumSize(Dimension(width.toInt, height.toInt))
     case _ => ()
   }
   pack()

java2d/src/main/scala/doodle/java2d/examples/EmptyFrame.scala

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2015 Creative Scala
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package doodle.java2d.examples
+
+import cats.effect.*
+import doodle.core.*
+import doodle.java2d.*
+import doodle.java2d.effect.Frame
+import doodle.syntax.all.*
+
+// This just renders an empty frame, so we can check it renders correctly.
+//
+// See issue #179
+object EmptyFrame extends IOApp {
+  def run(args: List[String]): IO[ExitCode] = {
+    val frame = Frame.default
+      .withSize(width = 300, height = 300)
+      .withBackground(Color.midnightBlue)
+    frame
+      .canvas()
+      .use { canvas =>
+        canvas.closed >> IO.pure(ExitCode.Success)
+      }
+  }
+}