internal/egl: replace glFinish with eglWaitClient

glFinish depends on package gl which is about to require a function
pointer loader. eglWaitClient is in the EGL API and always available.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/egl/egl_unix.go

@@ -102,3 +102,7 @@ func eglCreateWindowSurface(disp _EGLDisplay, conf _EGLConfig, win NativeWindowT
 	eglSurf := C.eglCreateWindowSurface(disp, conf, win, &attribs[0])
 	return eglSurf
 }
+
+func eglWaitClient() bool {
+	return C.eglWaitClient() == C.EGL_TRUE
+}