Improve audio waveform scrolling performance (#1292)

The object is already cached & the waveform view constructed when scrolling,
so there's no need to re-create the view.

This doesn't fix the problem with the wave-form generation on the first
view of the message.  That is a more critical problem in the underlying
SCWaveformView class which renders on the main thread instead of doing
it asynchronously.

FIXES #1258

Signal/src/Models/TSMessageAdapaters/TSVideoAttachmentAdapter.m

@@ -143,14 +143,16 @@
         NSURL *url =
             [MIMETypeUtil simLinkCorrectExtensionOfFile:_attachment.mediaURL ofMIMEType:_attachment.contentType];
 
+        if (!self.waveform) {
             AVURLAsset *asset         = [[AVURLAsset alloc] initWithURL:url options:nil];
-        _waveform                 = [[SCWaveformView alloc] init];
+            self.waveform                 = [[SCWaveformView alloc] init];
-        _waveform.frame           = CGRectMake(42.0, 0.0, size.width - 84, size.height);
+            self.waveform.frame           = CGRectMake(42.0, 0.0, size.width - 84, size.height);
-        _waveform.asset           = asset;
+            self.waveform.asset           = asset;
-        _waveform.progressColor   = [UIColor whiteColor];
+            self.waveform.progressColor   = [UIColor whiteColor];
-        _waveform.backgroundColor = [UIColor colorWithRed:229 / 255.0f green:228 / 255.0f blue:234 / 255.0f alpha:1.0f];
+            self.waveform.backgroundColor = [UIColor colorWithRed:229 / 255.0f green:228 / 255.0f blue:234 / 255.0f alpha:1.0f];
-        [_waveform generateWaveforms];
+            [self.waveform generateWaveforms];
-        _waveform.progress = 0.0;
+            self.waveform.progress = 0.0;
+        }
 
         _audioBubble = [[UIView alloc] initWithFrame:CGRectMake(0.0, 0.0, size.width, size.height)];
         _audioBubble.backgroundColor =