Only restore gamma ramp if it has been changed.

src/internal.h

diff --git a/src/internal.h b/src/internal.h
index 6398bdc..681df74 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -245,6 +245,7 @@
     GLFWgammaramp currentRamp;
     GLFWgammaramp originalRamp;
     int           originalRampSize;
+    GLboolean     rampChanged;
 
     // This is defined in the current port's platform.h
     _GLFW_PLATFORM_LIBRARY_WINDOW_STATE;