gladegl.cpp: fixes for C code as cpp

[puffnfresh]

For some reason this file is .cpp (and for me compiled with g++) but it didn't have the right headers and the rest of the code expected it to export C calls.

Alternatively, we could change the build and make this just C.

[devernay]

• never use extern "C" { before includes
• I don't see what's wrong with the previous code, unless you were including gladegl.h from C source files. This is all C++, so let's keep it that way
• The only valid change would be to error if gladegl.h is included from C

[puffnfresh]

@devernay no, it's not just from including in C files, the use is here:

openfx-supportext/ofxsOGLUtilities.cpp

Lines 42 to 45 in 62209f7

	extern "C" {
	extern int gladLoadGL(void);
	#if !defined(_WIN32) && !defined(__CYGWIN__) && !defined(__APPLE__) && !defined(__HAIKU__)
	extern int gladLoadEGL(void);

It's an explicit extern "C". I don't know why but it's consistent with gladLoadGL so I didn't want to change it.

I even question why it's an extern, can't we include gladegl.h? Nothing else does! Very confusing.

[f-dy]

The proper solution would be to declare it as extern c in the header file, and include the header file everywhere it's being used