Support Video Codecs with Alpha Channels + Bonus Fix

[hx2A]

• Video codecs such as Prores 4444 can contain an alpha channel, and it would be great if this library could support them. It is a small fix and works great.
• Added implementations of copy() and get(x, y, w, h) to both Movie and Class. I saw the get(x, y) method checks outdatedPixels before calling the parent method. The copy() and get(x, y, w, h) implementations inherit from PImage but should not go to the parent class without also checking outdatedPixels first. There's also a get() method in the parent but that is deprecated. I left it out; should I include it in this PR?

[hx2A]

Apple Prores 4444 and Apple Animation are both available video codecs that support alpha channels. If needed I can provide an example file for testing.