Implement image file support

I implemented the ability to use either FFmpeg or MagickCore in to libobs to allow the ability to create textures from image files, though MagickCore will need some extra cmake wizardry in order to be used if FFmpeg isn't available.  Right now it's just an unused file, though I tested it and it works fine (that I could see).

Filled in the rest of the 'path' plugin property code (so the image source module can use it to allow the user to select a file).

Added an image source that uses the new file loading function.