gsk: Clear current context after unrealize()

Make sure both GL renderers don't leave their contexts alive via the
current context, but ensure they dispose of them properly.

Fixes issues when the corresponding GL resources in the surfaces they
were attached to go away.

gsk/gl/gskglrenderer.c

@@ -258,6 +258,8 @@ gsk_gl_renderer_unrealize (GskRenderer *renderer)
   g_clear_object (&self->driver);
   g_clear_object (&self->command_queue);
   g_clear_object (&self->context);
+
+  gdk_gl_context_clear_current ();
 }
 
 static cairo_region_t *

gsk/gpu/gsknglrenderer.c

@@ -139,6 +139,8 @@ gsk_ngl_renderer_unrealize (GskRenderer *renderer)
 
   gsk_ngl_renderer_free_backbuffer (self);
 
+  gdk_gl_context_clear_current ();
+
   GSK_RENDERER_CLASS (gsk_ngl_renderer_parent_class)->unrealize (renderer);
 }