[host] dxgi: sleep until it's close to time to map

This change adds an average function to time how long it takes the GPU
to copy and map the texture, and then uses this average to sleep for 80%
of this average lowering CPU usage and potentially decreasing lock
contention.

common/include/common/runningavg.h

@@ -0,0 +1,28 @@
+/*
+Looking Glass
+Copyright (C) 2017-2021 The Looking Glass Authors
+https://looking-glass.io
+
+This program is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free Software
+Foundation; either version 2 of the License, or (at your option) any later
+version.
+
+This program is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+PARTICULAR PURPOSE. See the GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+this program; if not, write to the Free Software Foundation, Inc., 59 Temple
+Place, Suite 330, Boston, MA 02111-1307 USA
+*/
+
+#include <stdint.h>
+
+typedef struct RunningAvg * RunningAvg;
+
+RunningAvg runningavg_new(int length);
+void runningavg_free(RunningAvg * ra);
+void runningavg_push(RunningAvg ra, int64_t value);
+void runningavg_reset(RunningAvg ra);
+double runningavg_calc(RunningAvg ra);