Playstation Vita port

[devnoname120]

Here are changes allowing to run Julius on a Playstation Vita device.

Screenshots:

Notes:

• Lots of commits, because I tried different approaches (e.g. SDL for rendering, then switched to a native Vita library for performance reasons). I can squash them once this pull request is reviewed.
• @rsn887 did a pull request on top of my changes in order to improve input handling (vita: multiple control enhancements (see below) devnoname120/julius#1). This pull request will be done on https://github.com/bvschaik/julius instead when mine is merged.
• Performances are pretty good, the game is playable even in big cities. The videos are laggy however.
• I had to create a wrapper on top of fopen() because Julius relies a lot on chdir()/getcwd() which are not supported on the Vita.
• It should be possible to automatically compile using Travis with the vitasdk, there is also a Docker container. However, it will not be possible to run automated tests since there is no reliable PS Vita emulator yet. A physical PS Vita running a special program receiving unit tests could theoretically be done but it's a lot of work and a maintenance burden.

[codecov-io]

Codecov Report

Merging #110 into master will increase coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   63.48%   63.49%   +<.01%     
==========================================
  Files         174      174              
  Lines       25182    25188       +6     
==========================================
+ Hits        15988    15994       +6     
  Misses       9194     9194

Impacted Files	Coverage Δ
src/game/settings.c	63.68% <ø> (ø)	⬆️
test/sav/sav_compare.c	73.07% <ø> (ø)	⬆️
test/sav/run.c	64.28% <ø> (ø)	⬆️
src/core/io.c	89.18% <100%> (ø)	⬆️
src/core/file.c	14.63% <100%> (+14.63%)	⬆️
src/game/file_io.c	82.87% <66.66%> (ø)	⬆️
src/core/dir.c	36.06% <66.66%> (ø)	⬆️

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