EGL: Add support for EGL_EXT_platform_base

This adds support for EGL_EXT_platform_base and its associated X11 and
Wayland extensions, allowing us to explicitly tell EGL which window
system we are using.

This is based on work by @linkmauve in #1691.

Closes #1691.

src/internal.h

@@ -681,6 +681,7 @@ void _glfwPlatformWaitEvents(void);
 void _glfwPlatformWaitEventsTimeout(double timeout);
 void _glfwPlatformPostEmptyEvent(void);
 
+EGLenum _glfwPlatformGetEGLPlatform(void);
 EGLNativeDisplayType _glfwPlatformGetEGLNativeDisplay(void);
 EGLNativeWindowType _glfwPlatformGetEGLNativeWindow(_GLFWwindow* window);