XySubFilter support

Adds support for the subtitle interface used by XySubFilter to the VMR-9 and EVR-CP video renderers. It also includes an aspect-ratio fix by @Underground78 for PGS subtitles, this fix is required to support the subtitle resolutions offered by XySubFilter as they are also relative to the video frame instead of the video window.

For external subtitle files we should add subtitle source filter on the filter graph instead of auto-loading XySubFilter. The advantage of this approach is that we can let DirectShow handle loading and connecting XySubFilter just like any other filter. This functionality requires that "auto-load subtitles" is enabled and the ISR is disabled (ISR is always disabled atm until a seperate option is introduced). The downside of this method is that we need to support reading all the formats that XySubFilter supports. A unfinished version of this filter is already present in the repo under SubtitleSource which currently supports: `srt; ass; ssa; usf`.

Another ongoing issue is that the subtitle interface XySubFilter uses doesn't support invalidation. This means that when it switches subtitle streams the subtitles from the previous stream still in the queue will still be shown until they are removed.

### Points of discussion

- [ ] Should we offer external subtitles on the graph or auto-load XySubFilter and all other registered subtitle filters?
- [ ] Should we introduce a separate option to disable the ISR or always offer subtitles on the graph when auto-load subtitles is disabled?
- [x] Should we disable the subtitle queue for XySubFilter or request invalidation support in the interface? _Disable it for now_

### To-do list

**First merge:**
- [x] Support embedded subtitles rendered by XySubFilter.

**Second merge:**
- [ ] Support subtitle clipping for the ISR/XySubFilter.
- [ ] Support external subtitles through SubtitleSource.

**Third merge:**
- [ ] Enable subtitle queue once invalidation is supported.
- [ ] Support multiple bitmaps instead of relying on `combineBitmaps`. This is not yet necessary for XySubFilter support as there is currently no case in which it supplies more than one bitmap even without that flag.

**Subtitle interface:**
- [ ] Needs subtitle invalidation support so the subtitle queue is cleared when the subtitle stream is changed.
- [ ] Allow the consumer to control which subtitle sources are used by the subtitle provider. Needed to disable external subtitle loading in the provider if the player provides them through a source filter.

**XySubFilter Patch**
https://dl.dropboxusercontent.com/u/10853398/remove_workaround.patch

### Test builds

Contains custom LAV build with https://github.com/jeeb/lavfilters/commit/4097af9e822f1277deb98c9af238cd8fe2935a3f.

https://www.dropbox.com/sh/xl0f1tjvoz0ywm1/ASuyyGIAi7

Works with XySubFilter_3.1.0.546_BETA