[client] opengl: make ds functions optional

Using a macro ENABLE_OPENGL just like ENABLE_EGL to optionally remove
OpenGL implementation code. This is mostly because on Wayland it's just
a rehash of the EGL code (as EGL is the only way to create OpenGL
contexts on Wayland).

client/src/app.c

@@ -583,6 +583,7 @@ void app_eglSwapBuffers(EGLDisplay display, EGLSurface surface)
 }
 #endif
 
+#ifdef ENABLE_OPENGL
 LG_DSGLContext app_glCreateContext(void)
 {
   return g_state.ds->glCreateContext();
@@ -607,6 +608,7 @@ void app_glSwapBuffers(void)
 {
   g_state.ds->glSwapBuffers();
 }
+#endif
 
 void app_alert(LG_MsgAlert type, const char * fmt, ...)
 {