Fix GCD computation for negative values and support all integral types.

Using the GCD function on negative numbers caused an infinite loop, instead return the greatest positive common factor.

Fixes #4285.
EVR-CP and Sync renderers: Correctly compute the aspect ratio.

Use 64-bit unsigned integer to avoid overflows.

Fixes #4285.
Updated LAV Filters to 0.61.2-31-49cb3e7 (custom build based on 0.61.2-26-523ad32)

Important changes:
 - LAV Video: Fix some performance regressions introduced in v0.61  (mostly visible on old operating systems like Windows XP) (fixes #4357).
 - Use Win32 threads instead of pthreads.
 - Updated ffmpeg and libbluray.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
ISR: Render text subtitles relative to the video frame when in doubt.
Why "C:\tmp.srt"?
Fix some CPPCheck warnings or mark them as intentional.
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...
Fix: x86 and x64 versions conflicted over file associations.

4f465cf8b81f1da800b1bfc57cc48e317d24fbb5 was incomplete for Vista+.

This commit fixes #4064.
Auto mode for relative to.