project: replaced proxy with pre-cache

Removed proxy task and replaced with a true honest-to-god pre-cache for
footage. This footage is pre-cached to a sequence and therefore 100% ready
for use in it once the task is done.

app/render/audioplaybackcache.cpp

@@ -52,7 +52,9 @@ void AudioPlaybackCache::SetParameters(const AudioParams &params)
 
   // Our current audio cache is unusable, so we truncate it automatically
   TimeRange invalidate_range(0, NoLockGetLength());
-  NoLockInvalidate(invalidate_range);
+  if (invalidate_range.in() != invalidate_range.out()) {
+    NoLockInvalidate(invalidate_range);
+  }
 
   locker.unlock();