Minor clang-tidy fixups in some of the "next" magnum-bindings

[Skylion007]

Apply some minor perf/correctness improvement to the next-bindings:

• Don't pass by value unused variables
• Explicitly default empty virtual dtor constructors. This allows the optimizer to be more aggressive in optimizing them.

[codecov]

Codecov Report

Merging #16 (f22d377) into next (732a0b7) will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##             next      #16   +/-   ##
=======================================
  Coverage   91.18%   91.18%           
=======================================
  Files          45       45           
  Lines        4709     4709           
=======================================
  Hits         4294     4294           
  Misses        415      415           

Impacted Files	Coverage Δ
src/python/magnum/platform/cgl.cpp	61.53% <0.00%> (ø)
src/python/magnum/platform/glfw.cpp	36.84% <0.00%> (ø)
src/python/magnum/platform/glx.cpp	61.53% <0.00%> (ø)
src/python/magnum/gl.cpp	95.10% <100.00%> (ø)
src/python/magnum/platform/egl.cpp	76.92% <100.00%> (ø)

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[mosra]

Thank you!