Limit the number of feature indices processed during feature collection.

[garretrieger]

To fix #2191 .

[garretrieger]

I set the limit pretty high, we can tune it down later if needed. Looking at a merged copy of Noto Sans which contains support for everything other than CJK it uses ~500 feature indices.

[blueshade7]

Looks good. Thanks.
Let's work on lookups later when we see a case timing out with lookups alone.

[ebraminio]

Please put the font that shows the issue on test/fuzzing/fonts :)

[garretrieger]

Done :)

[garretrieger]

Test case is still timing out, I'll try lowering the limit.

[garretrieger]

Did some profling, now a lot of time is being spent in closure features (as opposed to collect features), probably needs some limiting applied in there as well.

[garretrieger]

Now Script subsetting is the most expensive part, will need to apply feature limiting there too I suspect.

[garretrieger]

So with all the fixes, it goes from taking ~5s to run the fuzzer file to ~0.08s.