VideoCommon: Support dumping frames to images

[stenzek]

This enables support for dumping frames to a series of png files instead of an avi file.
Therefore, we can support frame dumping on Android, or other platforms where libav is not available.

I haven't tested this at all on Android, but I don't see why it shouldn't work.

Note: I haven't made any changes to the Android UI, if we were to add an option for it, it'd probably be best placed in menu that is accessible at runtime, rather than the main settings menu as you wouldn't want to leave it enabled for long, due to how many files would be created.

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This change is 

[degasus]

Do you want to update fifoci to use this new feature? I doubt there is a need to export everything at 60 fps in fifoci as AVIDump does. https://github.com/dolphin-emu/fifoci/blob/master/runner/linux/run_fifo_test.sh#L110

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Reviewed 4 of 4 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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