Animation: Render WebM

Added support for rendering animation as WebM video with configurable quality settings.

Requires not only ffmpeg, but the libvpx VP9 codec library.

Test Plan

Eoin and I need help testing this, particularly on Windows and OSX. Can we make sure that we have access to libvpx on Windows and OSX. (We'll make sure that it's also available in an appimage.)

Create an animation, select File > Render Animation, render as WebM, click the ... to open the configuration widget if you want, and render.

Other Points

@kamathraghavendra With the addition of WebM support, would it be possible to add WebM animation upload support to Krita-Artists? I noticed that we support WebP but not WebM. (It's no rush though!)

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.