GPUImageFilterGroup endProcessing
Silence "Variable Unused" warnings when assertions are blocked (Release builds)
Merge remote-tracking branch 'upstream/master'
updated iOS version reference to avoid explicit release of semaphores on iOS6
replaced dispatch_semaphore_create(1) with dispatch_semaphore_create(0) + signal
Do not start writing audio frames until at least one video frame has been written. Fixes an issue where first video frame in the movie is occasionally blank
Useless checking
Add a `progress` property for `GPUImageMovie` specifying the progress of the process.
Add usage of progress to SimpleVideoFileFilter example.
Fix FilterShowcase project

OS X-only headers were getting in the way.
`dispatch_release` excluded on 6.0+ (instead of 7.0+)

Changed `#if` wrapping around `dispatch_release` to check for iOS 6.0 or newer, rather than 7.0 or newer.

This fixes errors:
* `error: 'release' is unavailable: not available in automatic reference counting mode`
* `ARC forbids explicit message send of 'release'`

occurring on the `dispatch_release` call when compiling against iOS 6.x (checked on iOS 6.1; assumed the problem is 6.0+-relevant based on https://github.com/ReactiveCocoa/ReactiveCocoa/issues/204 and https://github.com/Nyx0uf/NYXImagesKit/issues/16).

Resolves BradLarson/GPUImage#1528
Movie processing made more stable

* Checking more statuses for GPUImageMovie, as reader can be failed or
cancelled, but this is not handled properly
* Checking inputs' statuses before appending buffers in 
GPUImageMovieWriter, also checking blocks before calling them
Add method to remove filter using only the filter
Change filter input and outputs to the base class
Merge pull request #1541 from tonyd256/td-pipeline-remove-filter

Add method to remove filter using only the filter
Merge pull request #1542 from tonyd256/td-pipeline-filter-class

Change filter input and outputs to the base class
Made the pipeline additions a little more generic, so they wouldn't break on the Mac.
Merge pull request #1529 from capnslipp/issues/BradLarson/1528-patch1

`dispatch_release` excluded on 6.0+ (instead of 7.0+)
Merge pull request #1525 from rivera-ernesto/Fix_FilterShowcase

Fix FilterShowcase project
Merge pull request #1513 from telly/empty-frame-fix

Avoid writing audio frames until at least one video frame has been written. This prevents "black" frames at the beginning of the result movie file.