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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 0.61.2-36-a6bd936 (custom build based on 0.61.2-31-0c8c081)

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.
Fix some CPPCheck warnings.
Thumbnails: Show an error message when the thumbnails would be too small.
Don't update the last window position when the option is disabled.

Avoid always modifying the settings for those who sync them.

Fixes #4377.
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"?
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.
Cosmetics: Use better IDs from the resource strings.