[smooth] Implement Bezier quadratic arc flattenning with DDA

Benchmarking shows that this provides a very slighty performance
boost when rendering fonts with lots of quadratic bezier arcs,
compared to the recursive arc splitting, but only when SSE2 is
available, or on 64-bit CPUs.

On a 2017 Core i5-7300U CPU on Linux/x86_64:

  ./ftbench -p -s10 -t5 -cb .../DroidSansFallbackFull.ttf

    Before: 4.033 us/op  (best of 5 runs for all numbers)
    After:  3.876 us/op

  ./ftbench -p -s60 -t5 -cb .../DroidSansFallbackFull.ttf

    Before: 13.467 us/op
    After:  13.385 us/op
2 files changed