GTK: add a no-op glXSwapBuffers call for apitrace.

Apitrace uses swapping buffers to determine frame boundaries; before
this commit, everything solvespace renders gets put into a single
frame. Since we don't use double-buffered rendering, the call does
nothing (and is legal to perform), but apitrace output becomes
readable.

.gitignore

@@ -2,7 +2,7 @@
 /build*/
 /test/**/*.diff.*
 /test/**/*.curr.*
-*.trace # OpenGL apitrace files
+*.trace
 /debian/tmp/
 /debian/*.log
 /debian/*.substvars

src/platform/gtkmain.cpp

@@ -324,6 +324,8 @@ protected:
         cr->paint();
         surface->finish();
 
+        glXSwapBuffers(_xdisplay, _xwindow);
+
         return true;
     }