build.gradle: set OS-dependent 'prism.order' property

For headless test task, 'prism.order' property is used to choose the
graph renderer to use. Currently, we specify this property to be 'sw'
for all platforms.

However, this property triggers a bug of openjdk-jfx with headless
mode [1]. This property will cause Java Runtime Error for Windows OS
including AppVeyor:

    # A fatal error has been detected by the Java Runtime Environment:
    #
    #  EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x00007ffd95b64879, pid=1476, tid=2640
    #
    # JRE version: OpenJDK Runtime Environment (11.0.1+13) (build 11.0.1+13)
    # Java VM: OpenJDK 64-Bit Server VM (11.0.1+13, mixed mode, tiered, compressed oops, g1 gc, windows-amd64)
    # Problematic frame:
    # C  [javafx_font.dll+0x4879]

This bug has been identified and will be fixed in future release [2].
Let's set 'prism.order' property to be 'd3d' on Windows OS but retain
it to be 'sw' on other platforms as a temporary workaround to solve
the problem.

[1] javafxports/openjdk-jfx#66
[2] javafxports/openjdk-jfx#66 (comment)

build.gradle

@@ -163,6 +163,9 @@ test {
     }
 
     doFirst {
+        if (SystemUtils.IS_OS_WINDOWS) {
+            System.load("C:\\Windows\\System32\\WindowsCodecs.dll")
+        }
         boolean runGuiTests = gradle.taskGraph.hasTask(guiTests)
         boolean runNonGuiTests = gradle.taskGraph.hasTask(nonGuiTests)
 
@@ -188,12 +191,18 @@ test {
 
 task headless {
     doLast {
+        String orderProperty
+        if (SystemUtils.IS_OS_WINDOWS) {
+            orderProperty = 'd3d'
+        } else {
+            orderProperty = 'sw'
+        }
         println 'Setting headless mode properties.'
         test {
             systemProperties = [
                 'testfx.robot': 'glass',
                 'testfx.headless': 'true',
-                'prism.order': 'sw',
+                'prism.order': orderProperty,
                 'prism.text': 't2k',
             ]
         }