glspectrum: keep the OpenGL context current

There is no need to disable and enable the context between each
rendering. Since we own both the context and the thread, we can just
keep it as current until the thread is closed, simplifying the already
missing error handling.

modules/visualization/glspectrum.c

@@ -439,15 +439,12 @@ static void *Thread( void *p_data )
     struct glspectrum_opengl_vtable *vt = &p_sys->vt;
 
     initOpenGLScene(p_filter);
-    vlc_gl_ReleaseCurrent(gl);
-
     float height[NB_BANDS] = {0};
 
     while ((block = vlc_queue_DequeueKillable(&p_sys->queue, &p_sys->dead)))
     {
         unsigned win_width, win_height;
 
-        vlc_gl_MakeCurrent(gl);
         if (vlc_gl_surface_CheckSize(gl, &win_width, &win_height))
             vt->Viewport(0, 0, win_width, win_height);
 
@@ -573,9 +570,9 @@ static void *Thread( void *p_data )
 release:
         window_close(&wind_ctx);
         fft_close(p_state);
-        vlc_gl_ReleaseCurrent(gl);
         block_Release(block);
     }
+    vlc_gl_ReleaseCurrent(gl);
 
     return NULL;
 }