qemu-system-x86_64 -display sdl,gl=on

SDL1 display code has no opengl support.
Please recompile qemu with SDL2, using
./configure --enable-sdl --with-sdlabi=2.0
qemu-system-x86_64: OpenGL support is disabled

qemu-system-x86_64 -display sdl,gl=on

<graphics type='spice'>
  <listen type='none'/>
  <gl enable='yes'/>
</graphics>
<video>
  <model type='virtio'/>
</video>

error: Failed to start domain debian9-xfce
error: unsupported configuration: This QEMU doesn't support spice OpenGL

gir1.2-spice-client-gtk-3.0=0.33-3.3
libspice-server1=0.13.90-0.2
libvirglrenderer0=0.6.0-2
mesa-utils=8.3.0-5
libepoxy0=1.3.1-3

+    if (graphics->data.spice.gl) {
+        if (!virQEMUCapsGet(qemuCaps, QEMU_CAPS_SPICE_GL)) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("This QEMU doesn't support spice OpenGL"));
+            goto error;
+        }