auto-commit: translation updates
fix attempted double-translation ;) (thanks @shadmar)
run optipng on all PNG files to optimise and fix libPNG warnings

Recent versions of libpng (since 1.6 I think?) print a warning to
stderr when loading most of our PNGs. The warning is:

libpng warning: iCCP: known incorrect sRGB profile

and for some PNGs:

libpng warning: Interlace handling should be turned on when using
png_read_image.

This commit runs optipng on all our PNG files with this command:

  optipng -o2 -nc -nb -i 0 -strip all $IMAGE

Which strips the profile data from the image and rewrites it.
This fixes all the warnings, at the cost of generating a huge
commit and touching all our files.
replace remaining references to English
return nil to Lua if the requested resrouce wasn't found
use global gray colour for speedlines

the mxe build was consistently initialising LINE_COLOR to 0,0,0,0. it
appears to be some sort of initialisation order thing but I can't
figure out why its doing it. this will do
auto-commit: translation updates
specialise LuaObject<SystemPath>::PushToLua to cache SystemPath objects

This is done to ensure that two equivalent SystemPath objects will in fact
refer to the same Lua userdata instance. That allows SystemPaths to be
used as keys in Lua tables, which is very useful.
now that Lua SystemPath instances are all interned, they don't need __eq

The default userdata equality should now work correctly, because two
SystemPath values will be the same instance iff they have the same data.
make SystemPath 'final' to prevent accidents
extract the Lua SystemPath blob code to keep it with the rest of SystemPath

This should make it easier to maintain because it puts the memcpy code
that requires detailed knowledge of the content of a SystemPath together
with the rest of the SystemPath implementation.
(One of the most basic tenets of writing maintainable code, so really I
should have done it this way *first*)
auto-commit: translation updates
Merge remote-tracking branch 'johnbartholomew/deduplicate-lua-systempath'
auto-commit: translation updates
vs2013 et al don't support constexpr
remove the constexpr stuff entirely
Merge remote-tracking branch 'fluffy/no_constexpr'
auto-commit: translation updates
Merge branch 'master' of https://github.com/pioneerspacesim/pioneer into Shields