api: add support for float or rational FPS [API CHANGE]

Since x265_param was changing anyway, I went ahead and changed inputBitDepth
to internalBitDepth, which has always been its real function.

X265_BUILD is bumped to 6

--HG--
extra : amend_source : 376d21d5cdf31d709add8f3624181e35150c80e3