cogl: Replace unused field with computed value

As in the commit mentioned below, we dropped the only place where
current_gl_draw_buffer is set. Which was still being used for glx and
ended up causing black screen with multi-monitors & nvidia.

Fixes: aa0600bf2 ("cogl: Remove Framebuffer:stereo-mode")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4323>

cogl/cogl/cogl-context-private.h

@@ -196,7 +196,6 @@ struct _CoglContext
   GLuint                  current_gl_program;
 
   gboolean current_gl_dither_enabled;
-  GLenum current_gl_draw_buffer;
 
   /* Clipping */
   /* TRUE if we have a valid clipping stack flushed. In that case

cogl/cogl/winsys/cogl-onscreen-glx.c

@@ -790,7 +790,7 @@ cogl_onscreen_glx_swap_region (CoglOnscreen    *onscreen,
                                       rect[0], rect[1], x2, y2,
                                       GL_COLOR_BUFFER_BIT, GL_NEAREST);
         }
-      context->glDrawBuffer (context->current_gl_draw_buffer);
+      context->glDrawBuffer (GL_BACK);
     }
 
   /* NB: unlike glXSwapBuffers, glXCopySubBuffer and