Add epoxy_has_glx()

Libraries and applications that depend on Epoxy currently have no way to
safely degrade functionality if they are used on a platform without GLX
support; the only way to achieve that is to perform a symbol check
themselves, by essentially copying what Epoxy already does.

By exposing `epoxy_has_glx()`, those libraries and applications now have
the chance of querying Epoxy itself and gracefully handle failure.

include/epoxy/glx.h

@@ -50,6 +50,7 @@ EPOXY_BEGIN_DECLS
 
 EPOXY_PUBLIC bool epoxy_has_glx_extension(Display *dpy, int screen, const char *extension);
 EPOXY_PUBLIC int epoxy_glx_version(Display *dpy, int screen);
+EPOXY_PUBLIC bool epoxy_has_glx(Display *dpy);
 
 EPOXY_END_DECLS
 

src/dispatch_glx.c

@@ -142,3 +142,27 @@ epoxy_has_glx_extension(Display *dpy, int screen, const char *ext)
      */
     return epoxy_extension_in_string(glXQueryExtensionsString(dpy, screen), ext);
 }
+
+/**
+ * @brief Checks whether GLX is available.
+ *
+ * @param dpy The X11 display
+ *
+ * @return `true` if GLX is available
+ */
+bool
+epoxy_has_glx(Display *dpy)
+{
+#if !PLATFORM_HAS_GLX
+    return false;
+#else
+    Bool (* pf_glXQueryExtension) (Display *, int *, int *);
+    int error_base, event_base;
+
+    pf_glXQueryExtension = epoxy_conservative_glx_dlsym("glXQueryExtension", false);
+    if (pf_glXQueryExtension && pf_glXQueryExtension(dpy, &error_base, &event_base))
+        return true;
+
+    return false;
+#endif /* !PLATFORM_HAS_GLX */
+}