Add glfw3impl.h, a single header implementation of GLFW for C/C++ (rebased) - #2890

This is the same as #2462, but rebased into the 3.5.1 release (WIP at the moment).

Some newly introduced functions might be yet to be renamed to the _glfw<function-name><platform>() pattern.

It has been tested only briefly on Linux for now. It builds successfully with both Wayland and X11 using CMake and also as a unity build. It also builds successfully as a Windows executable from a unity build and a Linux host.

At the moment, when building as C++ with Wayland support, warnings about the "visibility" attribute are shown. Additionally, there is an error related to the types used with the wl_array_for_each() macro. Apparently, this macro is not usable in C++ and the only alternative is to iterate the array manually.

Maybe we can set aside the plans to make the code a valid subset of C++ for now and open a separate issue about this.

These are the steps to test a unity build on Linux supporting both Wayland and X11:

1. Write a simple C program (named test.c in this example) that includes glfw3impl.h:

#include "include/GLFW/glfw3impl.h"

int main()
{
	return 0;
}

2. Generate Wayland headers:

wl_generate() {
	wayland-scanner client-header deps/wayland/$1.xml wayland-headers/$1-client-protocol.h
	wayland-scanner private-code deps/wayland/$1.xml wayland-headers/$1-client-protocol-code.h
}

rm -rf wayland-headers
mkdir wayland-headers

wl_generate wayland
wl_generate xdg-shell
wl_generate xdg-decoration-unstable-v1
wl_generate viewporter
wl_generate relative-pointer-unstable-v1
wl_generate pointer-constraints-unstable-v1
wl_generate fractional-scale-v1
wl_generate xdg-activation-v1
wl_generate idle-inhibit-unstable-v1

3. Build it as C using GCC:

gcc -o test.elf -lm -D_GLFW_X11 -D_GLFW_WAYLAND -I./wayland-headers test.c

4. To check if is also valid as a subset of C++, build it using G++:

gcc -o test.elf -lm -D_GLFW_X11 -D_GLFW_WAYLAND -I./wayland-headers test.c

If MinGW is installed, it can also be built as a Windows executable from a Linux host:

x86_64-w64-mingw32-gcc test.c -o test.exe -lopengl32 -lgdi32 -lwinmm
x86_64-w64-mingw32-g++ test.c -o test.exe -lopengl32 -lgdi32 -lwinmm