Attempt to reduce jar size a little

- Run optipng on all our images. This has very little effect on most of
   them (as they're all so small anyway), but has resulted in a 50%
   reduction in some cases.
 - Run Proguard on our shadowed dependencies (Cobalt).
 - Minify our JSON files, stripping all whitespace. This is mostly
   useful for FML's annotation cache, as that's a massive file, but
   still a semi-useful optimisation to make.

This has helped reduce the jar by about 110kb, which isn't much but
still feels somewhat worth it.

build.gradle

@@ -9,7 +9,9 @@ buildscript {
         }
     }
     dependencies {
+        classpath 'com.google.code.gson:gson:2.8.1'
         classpath 'net.minecraftforge.gradle:ForgeGradle:2.3-SNAPSHOT'
+        classpath 'net.sf.proguard:proguard-gradle:6.1.0beta1'
         classpath 'org.ajoberstar.grgit:grgit-gradle:3.0.0'
     }
 }
@@ -93,7 +95,60 @@ jar {
     from configurations.shade.collect { it.isDirectory() ? it : zipTree(it) }
 }
 
+import java.nio.charset.StandardCharsets
+import java.nio.file.*
+import java.util.zip.*
+
+import com.google.gson.GsonBuilder
+import com.google.gson.JsonElement
 import org.ajoberstar.grgit.Grgit
+import proguard.gradle.ProGuardTask
+
+task proguard(type: ProGuardTask, dependsOn: jar) {
+    description "Removes unused shadowed classes from the jar"
+
+    injars jar.archivePath
+    outjars "${jar.archivePath.absolutePath.replace(".jar", "")}-min.jar"
+
+    // We want to avoid as much obfuscation as possible. We're only doing this to shrink code size.
+    dontobfuscate; dontoptimize; keepattributes; keepparameternames
+
+    // Proguard will remove directories by default, but that breaks JarMount.
+    keepdirectories 'assets/computercraft/lua**'
+
+    // Preserve ComputerCraft classes - we only want to strip shadowed files.
+    keep 'class dan200.computercraft.** { *; }'
+
+    // Preserve the constructors in Cobalt library class, as we init them via reflection
+    keepclassmembers 'class org.squiddev.cobalt.lib.** { <init>(...); }'
+}
+
+gradle.projectsEvaluated {
+    tasks.withType(ProGuardTask) {
+        group "compact"
+
+        // Add the main runtime jar and all non-shadowed dependencies
+        libraryjars "${System.getProperty('java.home')}/lib/rt.jar"
+        sourceSets.main.compileClasspath
+            .filter { !it.name.contains("Cobalt") }
+            .each { libraryjars it }
+    }
+}
+
+task proguardMove(dependsOn: proguard) {
+    description "Replace the original jar with the minified version"
+    group "compact"
+
+    doLast {
+        Files.move(
+            file("${jar.archivePath.absolutePath.replace(".jar", "")}-min.jar").toPath(),
+            file(jar.archivePath).toPath(),
+            StandardCopyOption.REPLACE_EXISTING
+        )
+    }
+}
+
+reobfJar.dependsOn proguardMove
 
 processResources {
     inputs.property "version", project.version
@@ -129,6 +184,47 @@ processResources {
     }
 }
 
+task compressJson(dependsOn: extractAnnotationsJar) {
+    group "compact"
+    description "Minifies all JSON files, stripping whitespace"
+
+    def jarPath = file(jar.archivePath)
+
+    def tempPath = File.createTempFile("input", ".jar", temporaryDir)
+    tempPath.deleteOnExit()
+
+    def gson = new GsonBuilder().create()
+
+    doLast {
+        // Copy over all files in the current jar to the new one, running json files from GSON. As pretty printing
+        // is turned off, they should be minified.
+        new ZipFile(jarPath).withCloseable { inJar ->
+            new ZipOutputStream(new BufferedOutputStream(new FileOutputStream(tempPath))).withCloseable { outJar ->
+                inJar.entries().each { entry ->
+                    if(entry.directory) {
+                        outJar.putNextEntry(entry)
+                    } else if(!entry.name.endsWith(".json")) {
+                        outJar.putNextEntry(entry)
+                        inJar.getInputStream(entry).withCloseable { outJar << it }
+                    } else {
+                        ZipEntry newEntry = new ZipEntry(entry.name)
+                        newEntry.setTime(entry.time)
+                        outJar.putNextEntry(newEntry)
+
+                        def element = inJar.getInputStream(entry).withCloseable { gson.fromJson(it.newReader("UTF8"), JsonElement.class) }
+                        outJar.write(gson.toJson(element).getBytes(StandardCharsets.UTF_8))
+                    }
+                }
+
+            }
+        }
+
+        // And replace the original jar again
+        Files.move(tempPath.toPath(), jarPath.toPath(), StandardCopyOption.REPLACE_EXISTING)
+    }
+}
+
+assemble.dependsOn compressJson
 
 curseforge {
     apiKey = project.hasProperty('curseForgeApiKey') ? project.curseForgeApiKey : ''