Add sequential loading flag

[dapetcu21]

Sometimes, you might want to wait for the previous scene to fully transition out before transitioning the new scene in (a fade-to-black transition is such an example). This also has the benefit of not having both scenes in memory at the same time and masking the few slow loading frames of a scene in-between the two scenes (on a black screen).

This is exactly how we did scene transitions in Interrogation (though we rolled our own scene manager there).

This was already happening when transitioning to the same scene ID, so I just added a flag to have that behaviour on-demand.

[codecov]

Codecov Report

Merging #62 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #62   +/-   ##
=======================================
  Coverage   83.29%   83.29%           
=======================================
  Files           4        4           
  Lines         838      838           
=======================================
  Hits          698      698           
  Misses        140      140           

Impacted Files	Coverage Δ
monarch/monarch.lua	86.55% <100.00%> (ø)

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[britzl]

Nice little addition. Thanks!