egl: implement SDL_GL_EGL_PLATFORM attribute

This implements a new SDL_GL_EGL_PLATFORM attribute to set the
"platform" argument for SDL_EGL_LoadLibrary on Windows, macOS, and
Linux. I've limited it to those three operating systems because that's
what I've been able to test against.

include/SDL_video.h

@@ -249,7 +249,8 @@ typedef enum
     SDL_GL_CONTEXT_RELEASE_BEHAVIOR,
     SDL_GL_CONTEXT_RESET_NOTIFICATION,
     SDL_GL_CONTEXT_NO_ERROR,
-    SDL_GL_FLOATBUFFERS
+    SDL_GL_FLOATBUFFERS,
+    SDL_GL_EGL_PLATFORM
 } SDL_GLattr;
 
 typedef enum