genesys: Enable gamma setting when contrast or brightness is adjusted

Brightness and contrast adjustments are handled via gamma tables and
thus gamma functionality is required for these settings to have any
effect.

Previously if the device has a sensor with identity gamma
(gamma = {1, 1, 1}), then gamma tables were turned off, consequently
brightness and contrast settings had no effect.

The underlying issue was identified by Gunnar Hjalmarsson
<gunnarhj@ubuntu.com> and STK.

backend/genesys/low.cpp

@@ -638,11 +638,16 @@ bool should_enable_gamma(const ScanSession& session, const Genesys_Sensor& senso
     if ((session.params.flags & ScanFlag::DISABLE_GAMMA) != ScanFlag::NONE) {
         return false;
     }
+    if (session.params.depth == 16) {
+        return false;
+    }
+    if (session.params.brightness_adjustment != 0 || session.params.contrast_adjustment != 0) {
+        return true;
+    }
+
     if (sensor.gamma[0] == 1.0f || sensor.gamma[1] == 1.0f || sensor.gamma[2] == 1.0f) {
         return false;
     }
-    if (session.params.depth == 16)
-        return false;
 
     return true;
 }