[client] egl: fix out by one error with letterbox rendering

This replaces the scaled `destRect` with a version that uses doubles
correcting the rounding error that is causing a failure to properly
clear the black bar areas.

common/include/common/types.h

@@ -38,6 +38,11 @@ struct Rect
   int x, y, w, h;
 };
 
+struct DoubleRect
+{
+  double x, y, w, h;
+};
+
 struct Border
 {
   int left, top, right, bottom;