PERF: Use Boost.SmallVec in BCO interpolator

This avoids some per-pixel dynamic allocations. The effect is
surprisingly small, though:

Tested by upsampling by 4x a four band, 10980x10980 scene

before:
286.76user 5.17system 0:36.00elapsed 810%CPU (0avgtext+0avgdata 1013876maxresident)k
2288inputs+15071432outputs (5major+1442780minor)pagefaults 0swaps
290.62user 5.28system 0:34.09elapsed 867%CPU (0avgtext+0avgdata 1013312maxresident)k
35776inputs+15071432outputs (207major+1443014minor)pagefaults 0swaps
291.82user 5.01system 0:34.05elapsed 871%CPU (0avgtext+0avgdata 1013196maxresident)k
18960inputs+15071432outputs (133major+1443078minor)pagefaults 0swaps

after:
260.86user 5.44system 0:37.12elapsed 717%CPU (0avgtext+0avgdata 1013652maxresident)k
4272inputs+15071432outputs (3major+1443217minor)pagefaults 0swaps
257.90user 5.85system 0:38.22elapsed 690%CPU (0avgtext+0avgdata 1014272maxresident)k
8inputs+15071432outputs (1major+1443207minor)pagefaults 0swaps
261.12user 5.58system 0:36.90elapsed 722%CPU (0avgtext+0avgdata 1012968maxresident)k
0inputs+15071432outputs (0major+1443212minor)pagefaults 0swaps
21 jobs for !697 with bco-small-vec in 85 minutes and 36 seconds (queued for 11 minutes and 39 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #22106
fast-build

00:03:17

passed #22107
legal-check

00:01:30

 
  Prepare
passed #22109
centos-xdk-prepare

00:02:00

passed #22110
macos
macos-xdk-prepare

00:00:37

passed #22108
ubuntu-xdk-prepare

00:00:42

passed #22111
windows10
windows-10-prepare

00:01:46

passed #22112
windows8
windows-8-prepare

00:01:37

 
  Build
passed #22115
centos-xdk-build

00:28:16

passed #22113
debian-build

00:21:33

passed #22116
macos
macos-xdk-build

00:13:35

passed #22114
ubuntu-xdk-build-doc

00:36:31

passed #22119
ubuntu-xdk-qa-code-coverage

00:56:19

passed #22120
ubuntu-xdk-qa-static-analysis

00:29:36

passed #22117
windows10
windows-10-build

00:49:00

passed #22118
windows8
windows-8-build

00:43:20

 
  Report
passed #22121
ubuntu-xdk-report

00:05:24

 
  External
passed cdash:centos-6.6-gcc

passed cdash:debian-testing-gcc

passed cdash:ubuntu-18.04-fast

passed cdash:windows-10-x64-vc14.0

passed cdash:windows-8.1-x86-vc14.0