Revert "tools: Drop support for the gl renderer"

This reverts commit fd02afa2e4.

We don't want to remove the GL renderer from our tools yet, because we
use those tools for manual testing and having it available is useful.

In particular, reinstate the GL renderer for rendernode-tool benchmark.

tools/gtk-rendernode-tool-utils.c

@@ -89,6 +89,9 @@ get_renderer_for_name (const char *renderer_name)
 #endif
   else if (g_ascii_strcasecmp (renderer_name, "cairo") == 0)
     return gsk_cairo_renderer_new ();
+  else if (g_ascii_strcasecmp (renderer_name, "opengl") == 0 ||
+           g_ascii_strcasecmp (renderer_name, "gl") == 0)
+    return gsk_gl_renderer_new ();
   else if (g_ascii_strcasecmp (renderer_name, "ngl") == 0)
     return gsk_ngl_renderer_new ();
 #ifdef GDK_RENDERING_VULKAN