Fix Windows warnings about APIENTRY from GLAD library

[totalgee]

This is an alternative to my PR #2332, which didn't actually solve the problem with these GLAD warnings with MSVC, and not as "heavy" as updating to GLAD 2:

1>C:\Program Files (x86)\Windows Kits\10\Include\10.0.20348.0\shared\minwindef.h(130,1): warning C4005: 'APIENTRY': macro redefinition
1>(compiling source file '../../src/cinder/gl/wrapper.cpp')
1>C:\Users\holag\Documents\Work\...\cinder\include\glad\glad.h(518,1):
1>see previous definition of 'APIENTRY'

This happens when building with MSVC, depending on the include order of GLAD headers vs Windows headers that define APIENTRY. Adding the push/pop seems like a simple solution to the problem.

[totalgee]

Hmm, unfortunately this doesn't work on other platforms (see compile error e.g. on Ubuntu above). The macro (referenced by other macros) is undefined when it is needed later (compiling glad.c)...or something like that.