Automatically enable --fpr with high bit depth input

[negge]

Right now encoding a high bit depth y4m file fails silently with the nebulous error:

$ ./encoder_example tiny420p10.y4m -o tiny420p10.ogv File 'tiny420p10.y4m' is 2x2 25.000 fps 420p10 video. Failed to create encoder.

This is fixed by adding the --fpr flag. If depth > 8 is detected, we should automatically turn this feature on.

[danesh-d]

Seems to be relatively straightforward to do. Would it be OK if I work on it? Also, is there any y4m example file which I can use to reproduce the case?