CI: Various AppVeyor build speed improvements

CI: Speedup msbuild by using all CPU cores

CI: Retry failed downloads

Add AUTHORS file

Changes the OBS_FRONTEND_EVENT_SCENE_CHANGED event after transitioning
is complete rather than right when the user hits a scene change.

LGP devices are devices that induce anger in any sane developer because
they're prone to bad audio timestamps when using their decoded data
directly.  For that reason, add a hack that smooths the timestamps
within a large threshold to prevent audio skipping.

On windows, when a source has only audio (no video) yet is marked as
capable of both video and audio, it would be programmed to expect a
video frame to synchronize with.  This fixes that potential issue by
detecting whether any video is actually playing or not.

When kaspersky is installed on windows 7, FindWindowEx will fail to find
any windows due to apparently being blocked by kaspersky, so detect when
that happens, and fall back to GetWindow instead if it does.

Allows saving replays programmatically without having to use hotkeys.

Uses a custom library for checking whether game capture is active and
for killing stray obs processes

Changed the build scripts around so that it operates off of a different
directory structure

Similar to the OBS1 file where you can optionally get the signature if
available in the header (for auto-updating)

So many checkboxes were together almost senselessly.

If all retries fail, stop the build.

This is mainly to give visual feedback to those affected by NVENC not
working with older driver versions. Currenlty obs fails silently which
could go unnoticed for users who are using hotkeys as well as confuse
users who are not trained to read their logs when issues occur.

Closes jp9000/obs-studio#788

rtmp-services: Update twitch ingests

Adds canadian ingest server.

Due to crashes being caused by hardware acceleration in the media source
on mac, disable hardware acceleration of the media source being on by
default for the time being.

In transitions, because the 'to' and 'from' are always rendered to
textures, the end result will always have premultiplied alpha.  This
would cause alpha to have blackish edges during transition, and cause
semi-transparent images to appear darker than they were supposed to.

To replicate, simply set the transparency of a source to 50%, then
transition between that scene and another scene.  The source will appear
to "pop" in and out unnaturally due to the premultiplied alpha effect of
the render targets.

To fix this, the solution is to simply convert premultiplied alpha to
straight alpha in the transition pixel shaders.