Added visual test for multi-line label alignment.

Calculation of label alignment in multi-line labels seems to be
off. This test reproduces it by laying out a series of multi-line
labels along an S-shaped curve. In all these cases, the vertical
bar character should, to within a few pixels, line up. The
orientation at sharper corners is still a little off, but likely
due to discontinuities.
unhide the scons command run by make
Fix unaligned multi-line labels.

This forces offset lines to be aligned to the closest point to
the anchor point on the original line, meaning that they are
aligned where the offset line and original run parallel, or nearly
so.
Update tests affected by alignment change.

These are mostly text affected because it is multi-line. The
"lines-5" test case text is improved - it lines up now. The
"line-offset" test case is not multi-line, but the differences
appear to be minimal.
more consistent memset/memcpy usage
array allocation test
add std::string and std::valarray to array allocation benchmark
fix rendering of multiple styles with OGR plugin - refs #2048 and mapbox/tilemill#2202
fix #2141
check filesystem before trying to open plugin + only report unique directories searched - closes #2131

Conflicts:
	src/datasource_cache.cpp
only return true if plugins are actually newly registered
Merge pull request #2136 from MapQuest/mla-expr-v2-multi-line-text-alignment

Multi-line text alignment
now that we only support boost >= 1.47 we can remove older phoenix support
consistently use mapnik value types
iwyu
Merge branch 'master' of github.com:mapnik/mapnik into expr-v2
Merge branch 'expr-v2' of github.com:mapnik/mapnik into jmh-text-layouts

Conflicts:
	tests/visual_tests/test.py
Update test cases.