Add debug environment variable to disable image caching

Tusker.xcodeproj/xcshareddata/xcschemes/Tusker.xcscheme

@@ -89,6 +89,13 @@
             isEnabled = "YES">
          </CommandLineArgument>
       </CommandLineArguments>
+      <EnvironmentVariables>
+         <EnvironmentVariable
+            key = "DISABLE_IMAGE_CACHE"
+            value = "1"
+            isEnabled = "NO">
+         </EnvironmentVariable>
+      </EnvironmentVariables>
    </LaunchAction>
    <ProfileAction
       buildConfiguration = "Release"

Tusker/Caching/ImageCache.swift

@@ -16,6 +16,12 @@ class ImageCache {
     static let attachments = ImageCache(name: "Attachments", memoryExpiry: .seconds(60 * 2))
     static let emojis = ImageCache(name: "Emojis", memoryExpiry: .seconds(60 * 5), diskExpiry: .seconds(60 * 60))
     
+    #if DEBUG
+    private static let disableCaching = ProcessInfo.processInfo.environment.keys.contains("DISABLE_IMAGE_CACHE")
+    #else
+    private static let disableCaching = false
+    #endif
+
     private let cache: Cache<Data>
     
     private var groups = [URL: RequestGroup]()
@@ -38,7 +44,8 @@ class ImageCache {
     
     func get(_ url: URL, completion: ((Data?) -> Void)?) -> Request? {
         let key = url.absoluteString
-        if let data = try? cache.object(forKey: key) {
+        if !ImageCache.disableCaching,
+           let data = try? cache.object(forKey: key) {
             completion?(data)
             return nil
         } else {