Make OfxExport export a symbol on GCC and Clang, and use it in the examples

[garyo]

Summary

• OfxExport expanded to __declspec(dllexport) on Windows and to plain extern everywhere else, so on macOS and Linux it did nothing. A plugin built with hidden visibility (as the CMake build does, via CXX_VISIBILITY_PRESET hidden) exported no entry points through it, and every example carried its own eight-line EXPORT block to supply the visibility attribute.
• OfxExport now adds __attribute__((visibility("default"))) under GCC and Clang. The entry points OfxGetPlugin, OfxGetNumberOfPlugins and OfxSetHost are declared with it in ofxCore.h, and a plugin's definitions inherit the attribute from those declarations, so they are exported with no specifier of their own.
• The examples, and the Programming Guide's five example plugins under Documentation/sources/Guide/Code/, drop their EXPORT macros and define the entry points with no specifier, the pattern plugin authors can copy (18 files, net -122 lines). That also matches the guide's own snippets, which never showed a specifier. Support/Library keeps its own EXPORT/LOCAL pair for now, since it also uses LOCAL.
• The ColourSpace example's OfxSetHost took no parameter. In C++ that made it a separate overload with C++ linkage, exported under a mangled name (__Z10OfxSetHostv), so no host ever found or called it. It now takes the const OfxHost * the header declares.

Compatibility

• Plugins that already put OfxExport or their own visibility attribute on their definitions keep working: the attribute is simply repeated, which compilers accept silently.

Test plan

• Every example the CMake build produces (hidden visibility) exports exactly OfxGetPlugin and OfxGetNumberOfPlugins, plus OfxSetHost for ColourSpace, on macOS (checked with nm -gU). Overlay, which is not in the CMake plugin list, and the five guide examples checked the same way by hand; all build warning-free.
• Control: bare definitions of the three entry points against main's ofxCore.h export nothing under -fvisibility=hidden; against this branch they export exactly those three, in both C and C++, and an undeclared helper stays hidden.
• CI on Linux and Windows.

Assisted-by: Claude Code / Claude Fable 5.1, Claude Opus 5.5

[Guido-assim]

The function declarations for OfxGetPlugin, OfxGetNumberOfPlugins and OfxSetHost in ofxCore.h already have the OfxExport specifier, I don't think it is needed to have it again with the definitions in the cpp files.
The definition of OfxSetHost in colourspace.cpp seems to be missing the host argument.

[garyo]

Good catch on both, Guido. Updated the PR to remove the decls in the examples. Also I fixed the colourspace example (that was preventing OfxSetHost from getting called on that example altogether, since it's a different function, not even "export "C"").

[Guido-assim]

Looks good to me.

[Guido-assim]

Looks good to me.