Use get_input_channels() rather than MAX_INPUT_CHANNELS when populating network input array.

[Tilps]

(This is just a quick implementation of the fix KillerDucky proposed in #576)

[killerducky]

I was doing one too but let's use your. How about some comments like this:

+    // NNPlanes is a static structure that has room for the maximum
+    // number of planes (V1). So here use MAX_INPUT_CHANNELS.
+    // (It might be nice to make it dynamic or some other solution.)
+    //
+    // But the rest of the code should use get_input_channels()
+    // to find how many we are actually using.

[Tilps]

I added a comment.
'Very early, very quick tests' suggest this fix is at least +100Elo in self play at training type visit counts.

[dubslow]

I think our next priority needs to be a lot more testing code, that something like this went unnoticed for so long is not a good sign for the health of the codebase

[Tilps]

Testing still positive Elo, but down from previous highs. +40Elo after 35 games. I am using net 245, since I had it handy and thought that overtraining might obfuscate the win.