Add an option to keep the device awake

Add an option to prevent the device to sleep:

    scrcpy --stay-awake
    scrcpy -w

The initial state is restored on exit.

Fixes #631 <https://github.com/Genymobile/scrcpy/issues/631>

app/src/scrcpy.h

@@ -38,6 +38,7 @@ struct scrcpy_options {
     bool prefer_text;
     bool window_borderless;
     bool mipmaps;
+    bool stay_awake;
 };
 
 #define SCRCPY_OPTIONS_DEFAULT { \
@@ -72,6 +73,7 @@ struct scrcpy_options {
     .prefer_text = false, \
     .window_borderless = false, \
     .mipmaps = true, \
+    .stay_awake = false, \
 }
 
 bool