Propagate library defs to swig

[ferdnyc]

I suspect since the bindings were moved out of the src/ directory, the CMAKE_SWIG_FLAGS haven't been propagating to the swig command line that generates the wrapper script. This means that the #ifdefs in the openshot.i files are all interpreted wrong.

This PR adds generator expression logic to copy every public compile definition from the openshot library target over to the SWIG_FLAGS of the openshot.i file, which should ensure that the bindings are generated correctly.

In my version of CMake, all compile definitions are stored in the target property without the leading -D, even if it was originally set. So, the generator expression I added will prepend -D to each of the values. I'll have to check whether that holds for older CMake versions as well, otherwise we might end up with -D-DUSE_RESVG=1 or the like, which would be a problem. The CI runs should be a fair test of that, though. (And just in case, I took the -Ds out of the definitions we set, at least.)

Edit: That wasn't the issue, but older CMakes did have a problem with the generator expressions. Added a workaround using get_property(), tested back to CMake 3.5.

Also added a fix to FindResvg.cmake, for compatibility with older CMake versions.

[codecov]

Codecov Report

Merging #615 (0f01cc4) into develop (8bc959b) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #615      +/-   ##
===========================================
- Coverage    52.25%   52.24%   -0.01%     
===========================================
  Files          129      129              
  Lines        10772    10774       +2     
===========================================
  Hits          5629     5629              
- Misses        5143     5145       +2     

Impacted Files	Coverage Δ
src/ImageWriter.h	50.00% <0.00%> (-50.00%)	⬇️

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[ferdnyc]

Merging this, as the current state of affairs is quite broken, and it's part of the opencv branch now anyway.