For quite some time missions have had the ability to edit more than one background. This was previously used for fallback loading in case the first background's bitmaps were not available, but I realized it would be very handy to allow switching between backgrounds for in-mission jumps.
The FRED background dialog can now add and remove backgrounds from the list. Additionally, a new `change-background` sexp can instantly switch between backgrounds without messing around with bitmap references.
There is nothing specific to OpenGL in that code anymore since it's all
handled in the shaders at this point. This will make the lighting
information available in the normal graphics code which allows to use
those values when filling the uniform buffer for the model rendering.
The codebase had quite a few instances of .size() being called and then checked to see if it was 0 or not (or less than 1, or greater-than-or-equal-to 1, which are basically equivalent), when .empty() could be called instead (negated or not, depending on what the original condition was).
This introduces a new mechanism for handling bit flags. Instead of using an int (or two, or three) to write bits into, this uses the std::bitset in a wrapper class that accepts an enum class as input. This means that certain classes of errors we've had a few of in the engine, where a coder used the wrong set of flags or checked them against the wrong variable (SF_STEALTH vs SIF_STEALTH, for example) can now be detected at a compiler level rather than not at all.
Goober5000